Don't modify the host compiler's sysroot

This commit updates the building of the wasi-sdk sysroot to leverage the
`-resource-dir` argument from Clang to avoid modifying the host
compiler's sysroot with compiler-rt things. This should help improve the
experience of building a standalone sysroot with whatever host Clang is
on the system.

Closes #444

cmake/wasi-sdk-sysroot.cmake

@@ -11,13 +11,18 @@ option(WASI_SDK_DEBUG_PREFIX_MAP "Pass `-fdebug-prefix-map` for built artifacts"
 option(WASI_SDK_INCLUDE_TESTS "Whether or not to build tests by default" OFF)
 
 set(wasi_sysroot ${CMAKE_INSTALL_PREFIX}/share/wasi-sysroot)
+set(wasi_resource_dir ${CMAKE_INSTALL_PREFIX}/lib/clang/${clang_version})
+
+# Force usage of the custom-built resource-dir and sysroot for the rest of the
+# wasi compiles.
+add_compile_options(-resource-dir ${wasi_resource_dir})
+add_compile_options(--sysroot ${wasi_sysroot})
 
 if(WASI_SDK_DEBUG_PREFIX_MAP)
   add_compile_options(
     -fdebug-prefix-map=${CMAKE_CURRENT_SOURCE_DIR}=wasisdk://v${wasi_sdk_version})
 endif()
 
-
 # Default arguments for builds of cmake projects (mostly LLVM-based) to forward
 # along much of our own configuration into these projects.
 set(default_cmake_args
@@ -44,7 +49,6 @@ endif()
 # compiler-rt build logic
 # =============================================================================
 
-set(compiler_rt_dst ${CMAKE_INSTALL_PREFIX}/lib/clang/${clang_version})
 ExternalProject_Add(compiler-rt-build
   SOURCE_DIR "${llvm_proj_dir}/compiler-rt"
   CMAKE_ARGS
@@ -57,7 +61,7 @@ ExternalProject_Add(compiler-rt-build
       -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON
       -DCMAKE_C_COMPILER_TARGET=wasm32-wasi
       -DCOMPILER_RT_OS_DIR=wasi
-      -DCMAKE_INSTALL_PREFIX=${compiler_rt_dst}
+      -DCMAKE_INSTALL_PREFIX=${wasi_resource_dir}
   EXCLUDE_FROM_ALL ON
   USES_TERMINAL_CONFIGURE ON
   USES_TERMINAL_BUILD ON
@@ -68,26 +72,30 @@ ExternalProject_Add(compiler-rt-build
 # around some headers and make copies of the `wasi` directory as `wasip1` and
 # `wasip2`
 execute_process(
-  COMMAND ${CMAKE_C_COMPILER} -print-runtime-dir
-  OUTPUT_VARIABLE clang_runtime_dir
+  COMMAND ${CMAKE_C_COMPILER} -print-resource-dir
+  OUTPUT_VARIABLE clang_resource_dir
   OUTPUT_STRIP_TRAILING_WHITESPACE)
-cmake_path(GET clang_runtime_dir PARENT_PATH clang_runtime_libdir) # chop off `wasi`
-cmake_path(GET clang_runtime_libdir PARENT_PATH clang_sysroot_dir) # chop off `lib`
 add_custom_target(compiler-rt-post-build
+  # The `${wasi_resource_dir}` folder is going to get used as `-resource-dir`
+  # for future compiles. Copy the host compiler's own headers into this
+  # directory to ensure that all host-defined headers all work as well.
   COMMAND ${CMAKE_COMMAND} -E copy_directory
-    ${clang_sysroot_dir} ${compiler_rt_dst}
+    ${clang_resource_dir}/include ${wasi_resource_dir}/include
+
+  # Copy the `lib/wasi` folder to `libc/wasi{p1,p2}` to ensure that those
+  # OS-strings also work for looking up the compiler-rt.a file.
   COMMAND ${CMAKE_COMMAND} -E copy_directory
-    ${compiler_rt_dst}/lib/wasi ${compiler_rt_dst}/lib/wasip1
+    ${wasi_resource_dir}/lib/wasi ${wasi_resource_dir}/lib/wasip1
   COMMAND ${CMAKE_COMMAND} -E copy_directory
-    ${compiler_rt_dst}/lib/wasi ${compiler_rt_dst}/lib/wasip2
-  COMMAND ${CMAKE_COMMAND} -E copy_directory_if_different
-    ${compiler_rt_dst}/lib ${clang_runtime_libdir}
+    ${wasi_resource_dir}/lib/wasi ${wasi_resource_dir}/lib/wasip2
+
   COMMENT "finalizing compiler-rt installation"
 )
 add_dependencies(compiler-rt-post-build compiler-rt-build)
 
 add_custom_target(compiler-rt DEPENDS compiler-rt-build compiler-rt-post-build)
 
+
 # =============================================================================
 # wasi-libc build logic
 # =============================================================================
@@ -158,9 +166,9 @@ function(define_libcxx target)
 
   get_property(dir_compile_opts DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY COMPILE_OPTIONS)
   get_property(dir_link_opts DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY LINK_OPTIONS)
-  set(extra_cflags_list ${CMAKE_C_FLAGS} ${target_flags} --target=${target} ${dir_compile_opts} ${dir_link_opts})
+  set(extra_cflags_list ${CMAKE_C_FLAGS} ${target_flags} --target=${target} ${dir_compile_opts} ${dir_link_opts} -nodefaultlibs)
   list(JOIN extra_cflags_list " " extra_cflags)
-  set(extra_cxxflags_list ${CMAKE_CXX_FLAGS} ${target_flags} --target=${target} ${dir_compile_opts} ${dir_link_opts})
+  set(extra_cxxflags_list ${CMAKE_CXX_FLAGS} ${target_flags} --target=${target} ${dir_compile_opts} ${dir_link_opts} -nodefaultlibs)
   list(JOIN extra_cxxflags_list " " extra_cxxflags)
 
   ExternalProject_Add(libcxx-${target}-build