[memprof] Use COMPILER_RT_TEST_COMPILER

Unlike the other compiler-rt unit tests MemProf was not using the
`generate_compiler_rt_tests()` helper that ensures the test is compiled
using the test compiler (generally the Clang binary built earlier).
This was exposed by https://github.com/llvm/llvm-project/pull/83088
because it started adding Clang-specific flags to
COMPILER_RT_UNITTEST_CFLAGS if the compiler ID matched "Clang".

This change should fix the buildbots that compile compiler-rt using
a GCC compiler with LLVM_ENABLE_PROJECTS=compiler-rt.

Reviewed By: vitalybuka

Pull Request: https://github.com/llvm/llvm-project/pull/88074