[libc] Exclude few unused bits from x86 state for Windows

[hedingarcia]

Summary

Windows fenv_t does not include the MXCSR register and
the unused bits at the end of the x87 status. So we
exclude them in our struct definitions to make it
easy to read/write the state. getEnv and setEnv
were also excluded to avoid using MXCSR, but a
forthcoming patch will handle these functions.

[aeubanks]

libc/utils/FPUtil/x86_64/FEnv.h

94

according to https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros, we don't need to check for _WIN64, _WIN32 is enough

we should clean this up everywhere in libc

[hedingarcia]

Change macro directives to only evaluate if __WIN32 is undefined.

[sivachandra]

I think this is not correct but OK for now to unblock other work. We will revisit again when we enable fenv functions on Windows.

libc/utils/FPUtil/x86_64/FEnv.h

 }
 
 struct X87StateDescriptor {
   uint16_t ControlWord;
   uint16_t Unused1;
   uint16_t StatusWord;
   uint16_t Unused2;
   // TODO: Elaborate the remaining 20 bytes as required.
+#if !(defined(_WIN32))
   uint32_t _[5];
+#endif
 };
 
 struct FPState {
   X87StateDescriptor X87Status;
+#if !(defined(_WIN32))
   uint32_t MXCSR;
+#endif
 };
 
 static_assert(
     sizeof(fenv_t) == sizeof(FPState),
     "Internal floating point state does not match the public fenv_t type.");
 
 static inline uint16_t getX87ControlWord() {
   uint16_t w;
static inline int setRound(int mode) {
   mxcsrControl =
       (mxcsrControl & ~(0x3 << internal::MXCSRRoundingControlBitPosition)) |
       mxcsrValue;
   internal::writeMXCSR(mxcsrControl);
 
   return 0;
 }
 
+#if !(defined(_WIN32))
 static inline int getEnv(fenv_t *envp) {
   internal::FPState *state = reinterpret_cast<internal::FPState *>(envp);
   internal::getX87StateDescriptor(state->X87Status);
   state->MXCSR = internal::getMXCSR();
   return 0;
 }
 
 static inline int setEnv(const fenv_t *envp) {
   const internal::FPState *state =
       reinterpret_cast<const internal::FPState *>(envp);
   internal::writeX87StateDescriptor(state->X87Status);
   internal::writeMXCSR(state->MXCSR);
   return 0;
 }
+#endif
 
 } // namespace fputil
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_UTILS_FPUTIL_X86_64_FENV_H