builtins: adjust code to build for ARM64 on MSVC

Avoid the use of `__attribute__` syntax on MSVC as that is not portable.
For the weak symbol definition, use the MSVC extension of creating a
weak alias through `/alternatename`.

Author: compnerd

compiler-rt/lib/builtins/aarch64/fp_mode.c

@@ -23,9 +23,14 @@
 #ifndef __ARM_FP
 // For soft float targets, allow changing rounding mode by overriding the weak
 // __aarch64_fe_default_rmode symbol.
+#if defined(_MSC_VER)
+CRT_FE_ROUND_MODE __default_rmode = CRT_FE_TONEAREST;
+#pragma comment(linker, "/alternatename:__aarch64_fe_default_rmode=__default_rmode")
+#else
 CRT_FE_ROUND_MODE __attribute__((weak)) __aarch64_fe_default_rmode =
     CRT_FE_TONEAREST;
 #endif
+#endif
 
 CRT_FE_ROUND_MODE __fe_getround(void) {
 #ifdef __ARM_FP