[libc][msvc] fix mathlib build on WoA (#161258)

Fix build errors encountered when building math library on WoA.

1. Skip FEnv equality check for MSVC
2. Provide a placeholder type for vector types.

Author: SchrodingerZhu

libc/src/string/memory_utils/op_generic.h

@@ -41,12 +41,22 @@ static_assert((UINTPTR_MAX == 4294967295U) ||
               "We currently only support 32- or 64-bit platforms");
 
 #ifdef LIBC_COMPILER_IS_MSVC
-
+#ifdef LIBC_TARGET_ARCH_IS_X86
 namespace LIBC_NAMESPACE_DECL {
 using generic_v128 = __m128i;
 using generic_v256 = __m256i;
 using generic_v512 = __m512i;
 } // namespace LIBC_NAMESPACE_DECL
+#else
+// Special handling when target does not have real vector types.
+// We can potentially use uint8x16_t etc. However, MSVC does not provide
+// subscript operation.
+namespace LIBC_NAMESPACE_DECL {
+struct alignas(16) generic_v128 : public cpp::array<uint8_t, 16> {};
+struct alignas(32) generic_v256 : public cpp::array<uint8_t, 32> {};
+struct alignas(64) generic_v512 : public cpp::array<uint8_t, 64> {};
+} // namespace LIBC_NAMESPACE_DECL
+#endif
 
 #else
 namespace LIBC_NAMESPACE_DECL {
@@ -159,7 +169,8 @@ template <typename T> struct Memset {
 
   LIBC_INLINE static void block(Ptr dst, uint8_t value) {
     if constexpr (is_scalar_v<T> || is_vector_v<T>) {
-      store<T>(dst, splat<T>(value));
+      // Avoid ambiguous call due to ADL
+      generic::store<T>(dst, splat<T>(value));
     } else if constexpr (is_array_v<T>) {
       using value_type = typename T::value_type;
       const auto Splat = splat<value_type>(value);

libc/test/UnitTest/FEnvSafeTest.cpp

@@ -43,7 +43,7 @@ void FEnvSafeTest::set_fenv(const fenv_t &fenv) {
 
 void FEnvSafeTest::expect_fenv_eq(const fenv_t &before_fenv,
                                   const fenv_t &after_fenv) {
-#if defined(LIBC_TARGET_ARCH_IS_AARCH64)
+#if defined(LIBC_TARGET_ARCH_IS_AARCH64) && !defined(LIBC_COMPILER_IS_MSVC)
   using FPState = LIBC_NAMESPACE::fputil::FEnv::FPState;
   const FPState &before_state = reinterpret_cast<const FPState &>(before_fenv);
   const FPState &after_state = reinterpret_cast<const FPState &>(after_fenv);