sanitizer_common: Define FP_XSTATE_MAGIC1 for old glibc

D116208 (commit 1298273e82) added FP_XSTATE_MAGIC1.
However, when building with glibc < 2.16 for backward-dependency
compatibility, it is not defined - and the build breaks.

Note: The define comes from Linux's asm/sigcontext.h but the
file uses signal.h which includes glibc's bits/sigcontext.h - which
is synced from the kernel's file but lags behind.

Solution: For backward compatility with ancient systems, define
FP_XSTATE_MAGIC1 if undefined.

//For the old systems, we were building with Linux kernel 3.19 but to support really old glibc systems, we build with a sysroot of glibc 2.12. While our kernel (and the users' kernels) have FP_XSTATE_MAGIC1, glibc 2.12 is too old. – With this patch, building the sanitizer libs works again. This showed up for us today as GCC mainline/13 has now synced the sanitizer libs.//

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D124927

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp

@@ -218,6 +218,10 @@ namespace __sanitizer {
 
   unsigned ucontext_t_sz(void *ctx) {
 #    if SANITIZER_GLIBC && SANITIZER_X64
+    // Added in Linux kernel 3.4.0, merged to glibc in 2.16
+#      ifndef FP_XSTATE_MAGIC1
+#        define FP_XSTATE_MAGIC1 0x46505853U
+#      endif
     // 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