[compiler-rt][MSVC] Conditionally remove emupac.cpp for msvc (#149823)

#148094 introduces logic for emulated PAC, which utilizes language
extensions not available on MSVC.

Author: zacklj89

compiler-rt/lib/builtins/CMakeLists.txt

@@ -591,10 +591,14 @@ set(aarch64_SOURCES
   ${GENERIC_TF_SOURCES}
   ${GENERIC_SOURCES}
   cpu_model/aarch64.c
-  aarch64/emupac.cpp
   aarch64/fp_mode.c
 )
 
+# Append sources specific to AArch64 targets that aren't supported by MSVC.
+if(NOT MSVC)
+  list(APPEND aarch64_SOURCES aarch64/emupac.cpp)
+endif()
+
 set(COMPILER_RT_AARCH64_FMV_USES_GLOBAL_CONSTRUCTOR NOT(FUCHSIA OR APPLE))
 
 if (COMPILER_RT_HAS_AARCH64_SME)