Add option to include debug symbols

[Bobo1239]

(Basically the same as #72.)

Motivation for this is that compiling ripgrep fails with linker errors without that flag:
cargo xwin build --target x86_64-pc-windows-msvc

error: linking with `lld-link` failed: exit status: 1
[...]
  = note: lld-link: error: Cannot use debug info for 'libcmt.lib(std_type_info_static.obj)' [LNK4099]
          >>> failed to load reference 'd:\a01\_work\5\s\binaries\amd64ret\lib\amd64\libcmt.amd64.pdb': No such file or directory
[...]

In contrast (with this PR):
rm -r ~/.cache/cargo-xwin/
cargo xwin build --target x86_64-pc-windows-msvc --xwin-include-debug-symbols
build succeeds

Admittedly I have no idea what exactly causes debug symbols to become necessary for compilation here. Some other projects I've tested compile successfully without debug symbols.