[compiler-rt] [test] Add an .exe suffix for unit test executables on MinGW

[mstorsjo]

Summary

Previously, an .exe suffix was only added for MSVC configurations.

In practice, an .exe suffix is added implicitly by MinGW toolchains
if the output is a suffixless file name. However this can cause lots
of subtle build system confusion, when it's not generating the file it
expected.

[alvinhochun]

LGTM, this makes sense.

compiler-rt/cmake/Modules/AddCompilerRT.cmake

function(add_compiler_rt_test test_suite test_name arch)
   cmake_parse_arguments(TEST "" "SUBDIR" "OBJECTS;DEPS;LINK_FLAGS" "" ${ARGN})
   set(output_dir ${CMAKE_CURRENT_BINARY_DIR})
   if(TEST_SUBDIR)
     set(output_dir "${output_dir}/${TEST_SUBDIR}")
   endif()
   set(output_dir "${output_dir}/${CMAKE_CFG_INTDIR}")
   file(MAKE_DIRECTORY "${output_dir}")
   set(output_bin "${output_dir}/${test_name}")
-  if(MSVC)
+  if(WIN32)
     set(output_bin "${output_bin}.exe")
   endif()
 
   # Use host compiler in a standalone build, and just-built Clang otherwise.
   if(NOT COMPILER_RT_STANDALONE_BUILD)
     list(APPEND TEST_DEPS clang)
   endif()