[libc] Make add_libc_unittest compile with -ffreestanding

[gchatelet]

Summary

tests not compile with -ffreestanding can pull unwanted dependencies like limits.h which defines PTHREAD_STACK_MIN.
This is what caused the build bot failure in https://reviews.llvm.org/D156981#4570776.

libc/cmake/modules/LLVMLibCTestRules.cmake

function(create_libc_unittest fq_target_name)
   target_include_directories(
     ${fq_build_target_name}
     PRIVATE
       ${LIBC_SOURCE_DIR}
       ${LIBC_INCLUDE_DIR}
   )
   target_compile_options(
     ${fq_build_target_name}
-    PRIVATE -fpie ${LIBC_COMPILE_OPTIONS_DEFAULT}
+    PRIVATE -fpie -ffreestanding ${LIBC_COMPILE_OPTIONS_DEFAULT}
   )
   if(LIBC_UNITTEST_COMPILE_OPTIONS)
     target_compile_options(
       ${fq_build_target_name}
       PRIVATE ${LIBC_UNITTEST_COMPILE_OPTIONS}
     )
   endif()
   if(NOT LIBC_UNITTEST_CXX_STANDARD)