CRYPTO_MUTEX was the wrong size. Fortunately, Apple was kind enough to define
pthread_rwlock_t unconditionally, so we can be spared fighting with feature
macros. Some of the stdlib.h removals were wrong and clang is pick about
multiply-defined typedefs. Apparently that's a C11 thing?

BUG=478598

Change-Id: Ibdcb8de9e5d83ca28e4c55b2979177d1ef0f9721
Reviewed-on: https://boringssl-review.googlesource.com/4404
Reviewed-by: Adam Langley <agl@google.com>