[libc][NFC] Make x86_64 fenv functions msan safe.

These functions used inline asm to read FPU state. This change adds
explicit unpoisoning in these functions as the sanitizers don't see the
read operations.

libc/src/__support/CMakeLists.txt

@@ -2,4 +2,5 @@ add_header_library(
   common
   HDRS
     common.h
+    sanitizer_annotations.h
 )

libc/utils/FPUtil/CMakeLists.txt

@@ -31,6 +31,7 @@ add_header_library(
     libc.include.math
     libc.include.errno
     libc.include.fenv
+    libc.src.__support.common
     libc.utils.CPP.standalone_cpp
 )
 

libc/utils/FPUtil/x86_64/FEnv.h

@@ -12,6 +12,8 @@
 #include <fenv.h>
 #include <stdint.h>
 
+#include "src/__support/sanitizer_annotations.h"
+
 namespace __llvm_libc {
 namespace fputil {
 
@@ -95,6 +97,7 @@ static_assert(
 static inline uint16_t getX87ControlWord() {
   uint16_t w;
   __asm__ __volatile__("fnstcw %0" : "=m"(w)::);
+  SANITIZER_MEMORY_INITIALIZED(&w, sizeof(w));
   return w;
 }
 
@@ -105,6 +108,7 @@ static inline void writeX87ControlWord(uint16_t w) {
 static inline uint16_t getX87StatusWord() {
   uint16_t w;
   __asm__ __volatile__("fnstsw %0" : "=m"(w)::);
+  SANITIZER_MEMORY_INITIALIZED(&w, sizeof(w));
   return w;
 }
 
@@ -115,6 +119,7 @@ static inline void clearX87Exceptions() {
 static inline uint32_t getMXCSR() {
   uint32_t w;
   __asm__ __volatile__("stmxcsr %0" : "=m"(w)::);
+  SANITIZER_MEMORY_INITIALIZED(&w, sizeof(w));
   return w;
 }
 
@@ -124,6 +129,7 @@ static inline void writeMXCSR(uint32_t w) {
 
 static inline void getX87StateDescriptor(X87StateDescriptor &s) {
   __asm__ __volatile__("fnstenv %0" : "=m"(s));
+  SANITIZER_MEMORY_INITIALIZED(&s, sizeof(s));
 }
 
 static inline void writeX87StateDescriptor(const X87StateDescriptor &s) {