More throughly disable assertions. This works around USES=cmake
overriding the CFLAGS used to build. [0]

devel/llvm[45]0: Fix build on GCC systems. [1]

devel/llvm[456]: Avoid realpath calls where possible in the wrapper
script. [2]

PR: 233506 [0], 234647 [1], 234937 [1]
Submitted by: greg@unrelenting.technology [0], pkubaj@anongoth.pl [1],

		bdrewery [2]

Differential Revision: https://reviews.freebsd.org/D17990