[libc] Make add_libc_unittest compile with -ffreestanding

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.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157444