Update to the 20160816 snapshot of GCC 5.4.1.

GCC uses an AWK script to generate source code that helps process
command-line options. According to POSIX, string comparisons (and
hence sorting) are to be performed based on the locale's collating
order. Alas GNU AWK only does so in POSIX mode, whereas starting
with FreeBSD 11 we do so by default, running into a bug (or false
assumption) with that script used by GCC.

Setting MAKE_ARGS such that AWK is always invoked in the C locale
works around this bug. [1]

PR: 210122 [1], 211742 [1]
Submitted by: jkim [1]