[Sanitizers] Guard FP_XSTATE_MAGIC1 usage by GLIBC version

Follow-up on https://reviews.llvm.org/D118970 FP_XSTATE_MAGIC1 is only available on glibc 2.27 and upwards

Differential Revision: https://reviews.llvm.org/D124770

Author: vchuravy

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp

@@ -223,7 +223,7 @@ namespace __sanitizer {
   unsigned struct_sockaddr_sz = sizeof(struct sockaddr);
 
   unsigned ucontext_t_sz(void *ctx) {
-#    if SANITIZER_GLIBC && SANITIZER_X64
+#    if SANITIZER_GLIBC && SANITIZER_X64 &&  __GLIBC_PREREQ (2, 27)
     // Added in Linux kernel 3.4.0, merged to glibc in 2.16
 #      ifndef FP_XSTATE_MAGIC1
 #        define FP_XSTATE_MAGIC1 0x46505853U

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(__GLIBC__) && defined(__x86_64__)
+#if defined(__GLIBC__) && defined(__x86_64__) && __GLIBC_PREREQ(2, 27)
   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