[libc] Allows cross compilation of membenchmarks

[gchatelet]

Summary

This patch makes sure:

• we pass the correct compiler options when building Google benchmarks,
• we only import the C++ version of the memory functions.

The change in libc/cmake/modules/LLVMLibCTestRules.cmake is here to make sure CMake can generate the right command line in the presence of the CMAKE_CROSSCOMPILING_EMULATOR option.

Relevant documentation:
https://cmake.org/cmake/help/latest/variable/CMAKE_CROSSCOMPILING_EMULATOR.html
https://cmake.org/cmake/help/latest/command/add_custom_command.html#command:add_custom_command
"
If COMMAND specifies an executable target name (created by the add_executable() command), it will automatically be replaced by the location of the executable created at build time if either of the following is true:

• The target is not being cross-compiled (i.e. the CMAKE_CROSSCOMPILING variable is not set to true).
• New in version 3.6: The target is being cross-compiled and an emulator is provided (i.e. its CROSSCOMPILING_EMULATOR target property is set). In this case, the contents of CROSSCOMPILING_EMULATOR will be prepended to the command before the location of the target executable.

"

[gchatelet]

@sivachandra I'm submitting this. Let me know if you have any concerns.

[sivachandra]

LGTM

libc/benchmarks/CMakeLists.txt

ExternalProject_Add(google-benchmark-libc
         INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}/google-benchmark-libc
         CMAKE_CACHE_ARGS
           -DBENCHMARK_ENABLE_EXCEPTIONS:BOOL=OFF
           -DBENCHMARK_ENABLE_LTO:BOOL=OFF
           -DBENCHMARK_ENABLE_TESTING:BOOL=OFF
           -DBENCHMARK_ENABLE_WERROR:BOOL=${LLVM_ENABLE_WERROR}
           -DBENCHMARK_FORCE_WERROR:BOOL=OFF
           -DBENCHMARK_USE_LIBCXX:BOOL=OFF
-          -DCMAKE_BUILD_TYPE:STRING=RELEASE
+          -DCMAKE_BUILD_TYPE:STRING=Release
+
+          -DCMAKE_SYSTEM_NAME:STRING=${CMAKE_SYSTEM_NAME}
+          -DCMAKE_SYSTEM_PROCESSOR:STRING=${CMAKE_SYSTEM_PROCESSOR}
           -DCMAKE_C_COMPILER:STRING=${CMAKE_C_COMPILER}
           -DCMAKE_CXX_COMPILER:STRING=${CMAKE_CXX_COMPILER}
-          -DCMAKE_CXX_FLAGS:STRING=${BENCHMARK_LIBC_COMPILE_FLAGS}
+          -DCMAKE_CXX_FLAGS:STRING=${CMAKE_CXX_FLAGS}
+          -DCMAKE_FIND_ROOT_PATH:STRING=${CMAKE_FIND_ROOT_PATH}
+
+          -DBUILD_SHARED_LIBS:BOOL=OFF
+          -DCMAKE_EXE_LINKER_FLAGS:STRING=-static
+
           -DCMAKE_CXX_STANDARD:STRING=14
           -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
         )
 
 add_custom_target(libc-benchmark-util-tests)
 
 # libc-benchmark
 add_library(libc-benchmark

 add_executable(libc.benchmarks.memory_functions.opt_host
   EXCLUDE_FROM_ALL
   LibcMemoryGoogleBenchmarkMain.cpp
   LibcDefaultImplementations.cpp
 )
 target_link_libraries(libc.benchmarks.memory_functions.opt_host
   PRIVATE
   libc-memory-benchmark
-  libc.src.string.memcmp_opt_host
-  libc.src.string.bcmp_opt_host
-  libc.src.string.memcpy_opt_host
-  libc.src.string.memset_opt_host
-  libc.src.string.bzero_opt_host
-  libc.src.string.memmove_opt_host
+  libc.src.string.memcmp_opt_host.__internal__
+  libc.src.string.bcmp_opt_host.__internal__
+  libc.src.string.memcpy_opt_host.__internal__
+  libc.src.string.memset_opt_host.__internal__
+  libc.src.string.bzero_opt_host.__internal__
+  libc.src.string.memmove_opt_host.__internal__
   benchmark_main
+  -static
 )
 llvm_update_compile_flags(libc.benchmarks.memory_functions.opt_host)
 
 add_subdirectory(automemcpy)

libc/cmake/modules/LLVMLibCTestRules.cmake

function(create_libc_unittest fq_target_name)
   # LibcUnitTest should not depend on anything in LINK_LIBRARIES.
   list(APPEND link_libraries LibcUnitTestMain LibcUnitTest)
 
   target_link_libraries(${fq_build_target_name} PRIVATE ${link_libraries})
 
   if(NOT LIBC_UNITTEST_NO_RUN_POSTBUILD)
     add_custom_target(
       ${fq_target_name}
-      COMMAND $<TARGET_FILE:${fq_build_target_name}>
+      COMMAND ${fq_build_target_name}
       COMMENT "Running unit test ${fq_target_name}"
     )
   endif()
 
   if(LIBC_UNITTEST_SUITE)
     add_dependencies(
       ${LIBC_UNITTEST_SUITE}
       ${fq_target_name}