i#6953: Fix sigcontext test AVX error (#6954)

Replaces a kill() with an inlined system call in the linux.sigcontext
test to avoid failure when compiled for AVX only, as the compiler
inserts VZEROUPPER there (VZEROUPPER is recommended by Intel to avoid a
performance penalty when switching between upper-zeroing VEX-encoded XMM
operations and legacy upper-preserving SSE* operations: so doing it at
transitions to code you don't control makes sense).

Tested on a toolchain targeting AVX.

Fixes #6953

suite/tests/linux/sigcontext.c

@@ -35,6 +35,7 @@
 #include "tools.h"
 /* we want the latest defs so we can get at ymm state */
 #include "../../../core/unix/include/sigcontext.h"
+#include "../../../core/unix/include/syscall.h"
 #include <assert.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -183,6 +184,7 @@ main(int argc, char *argv[])
     int buf[INTS_PER_XMM * NUM_SIMD_SSE_AVX_REGS];
     char *ptr = (char *)buf;
     int i, j;
+    pid_t pid = getpid();
 
     /* do this first to avoid messing w/ xmm state */
     intercept_signal(SIGUSR1, signal_handler, false);
@@ -323,8 +325,21 @@ main(int argc, char *argv[])
         MOVE_TO_YMM(buf, 15)
 #        endif
 #    endif
-        /* now make sure they show up in signal context */
-        kill(getpid(), SIGUSR2);
+        /* Now make sure they show up in the signal context.
+         * But, it's not safe to make a regular call, as the compiler will
+         * insert VZEROUPPER for some configurations.
+         */
+#    ifdef X64
+        __asm__ __volatile__("mov %0, %%rdi; mov %1, %%rsi; mov %2, %%rax; syscall"
+                             :
+                             : "m"(pid), "i"(SIGUSR2), "i"(SYS_kill)
+                             : "rdi", "rsi", "rax");
+#    else
+        __asm__ __volatile__("mov %0, %%ebx; mov %1, %%ecx; mov %2, %%eax; int $0x80"
+                             :
+                             : "m"(pid), "i"(SIGUSR2), "i"(SYS_kill)
+                             : "ebx", "ecx", "eax");
+#    endif
 
         /* Ensure they are preserved across the sigreturn (xref i#3812). */
 #    ifdef __AVX512F__