sanitizer_common: Define FP_XSTATE_MAGIC1 for old glibc

[burnus]

Summary

D116208 (commit 1298273e8206a8fc2) 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.

[vitalybuka]

Maybe you rather should change build rules to disable sanitizer on such old config?
There is high probability that they are unusable there anyway.

[abidh]

Committed with slight whitespace change to keep clang-format happy.

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp

 #endif  // SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD ||
         // SANITIZER_MAC
 #if !SANITIZER_ANDROID
   unsigned struct_statfs_sz = sizeof(struct statfs);
   unsigned struct_sockaddr_sz = sizeof(struct sockaddr);
 
   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
     // is always the same.  So avoid using members, just use arithmetic.
     const uint32_t *after_xmm =
         reinterpret_cast<const uint32_t *>(fpregs + 1) - 24;
     if (after_xmm[12] == FP_XSTATE_MAGIC1)
       return reinterpret_cast<const char *>(fpregs) + after_xmm[13] -