[compiler-rt] Don't link builtins against the CRT on Windows (#70675)

compiler-rt/builtins doesn't depend on anything from the CRT but
currently links against it and embeds a `/defaultlib:msvcrt` in the
`.lib` file, forcing anyone linking against it to also link against that
specific CRT. This isn't necessary as the end user can just choose which
CRT they want to use independently.

Author: DavidTruby

compiler-rt/cmake/builtin-config-ix.cmake

@@ -18,6 +18,7 @@ builtin_check_c_compiler_flag(-fno-profile-generate COMPILER_RT_HAS_FNO_PROFILE_
 builtin_check_c_compiler_flag(-fno-profile-instr-generate COMPILER_RT_HAS_FNO_PROFILE_INSTR_GENERATE_FLAG)
 builtin_check_c_compiler_flag(-fno-profile-instr-use COMPILER_RT_HAS_FNO_PROFILE_INSTR_USE_FLAG)
 builtin_check_c_compiler_flag(-Wno-pedantic         COMPILER_RT_HAS_WNO_PEDANTIC)
+builtin_check_c_compiler_flag(/Zl COMPILER_RT_HAS_ZL_FLAG)
 
 builtin_check_c_compiler_source(COMPILER_RT_HAS_ATOMIC_KEYWORD
 "

compiler-rt/lib/builtins/CMakeLists.txt

@@ -761,6 +761,12 @@ else ()
 
   append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 BUILTIN_CFLAGS)
 
+  # Don't embed directives for picking any specific CRT
+  if (MSVC)
+    set(CMAKE_MSVC_RUNTIME_LIBRARY "")
+    append_list_if(COMPILER_RT_HAS_ZL_FLAG /Zl BUILTIN_CFLAGS)
+  endif()
+
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.
   if(COMPILER_RT_STANDALONE_BUILD)