Enable SUBDIR_PARELLEL in lib/libclang_rt

I noticed that this part of the build was taking much longer than
expected. Turns out it's due to not running the subdirs in parallel.
Reduces make all inside lib/libclang_rt time from 63s to 20s with -j32.

Reviewed By: dim
Differential Revision: https://reviews.freebsd.org/D26623