[rust] Define FP_XSTATE_MAGIC1 if not defined

It looks like on old linux versions (like our dist-x86_64-linux
builder) FP_XSTATE_MAGIC1 is defined only by asm/sigcontext.h,
but not by bits/sigcontext.h. We can't actually include
asm/sigcontext.h, because that would clash with types defined
in bits/sigcontext.h, which gets pulled in via signal.h.

Instead we just replicate the value from the header here.

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp

@@ -218,6 +218,9 @@ namespace __sanitizer {
 
   unsigned ucontext_t_sz(void *ctx) {
 #    if SANITIZER_GLIBC && SANITIZER_X64
+#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