[msan] Guard FP_XSTATE_MAGIC1 usage with SANITIZER_GLIBC

Fix build for Linux musl.

Reviewed By: #sanitizers, dvyukov

Differential Revision: https://reviews.llvm.org/D118970

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp

@@ -217,7 +217,7 @@ namespace __sanitizer {
   unsigned struct_sockaddr_sz = sizeof(struct sockaddr);
 
   unsigned ucontext_t_sz(void *ctx) {
-#    if SANITIZER_LINUX && SANITIZER_X64
+#    if SANITIZER_GLIBC && SANITIZER_X64
     // See kernel arch/x86/kernel/fpu/signal.c for details.
     const auto *fpregs = static_cast<ucontext_t *>(ctx)->uc_mcontext.fpregs;
     // The member names differ across header versions, but the actual layout

compiler-rt/test/msan/Linux/signal_mcontext.cpp

@@ -10,7 +10,7 @@
 
 void handler(int sig, siginfo_t *info, void *uctx) {
   __msan_check_mem_is_initialized(uctx, sizeof(ucontext_t));
-#if defined(__x86_64__)
+#if defined(__GLIBC__) && defined(__x86_64__)
   auto *mctx = &static_cast<ucontext_t *>(uctx)->uc_mcontext;
   if (auto *fpregs = mctx->fpregs) {
     // The member names differ across header versions, but the actual layout

compiler-rt/test/msan/Linux/signal_mcontext2.cpp

@@ -1,6 +1,7 @@
 // RUN: %clangxx_msan -fsanitize-memory-track-origins=2 -O1 %s -o %t && not %run %t 2>&1 | FileCheck %s
 
-// REQUIRES: x86_64-target-arch
+// musl does not define FP_XSTATE_MAGIC1.
+// REQUIRES: x86_64-target-arch && glibc-2.27
 
 #include <pthread.h>
 #include <signal.h>