libc/tests: Further refine the condition for installing h_raw

See 8164d511d6a6 ("libc/tests: Fix installation without MK_TOOLCHAIN")
for some background. Here we should really be testing MK_CLANG instead,
since that's what gates compilation of libclang_rt.

Fixes: 8164d511d6a6 ("libc/tests: Fix installation without MK_TOOLCHAIN")