MFC	r261178:

gcc: Add support for -Wmissing-prototypes in C++

Support for warnings about missing prototypes in C++ was added by Apple
GCC (Radar 6261539).  Most of the code crept into r260311 so it felt
natural to make use of it.

Obtained from:	Apple GCC - 5646
MFC	r261175:

binutils: add support for Intel SMAP-related instructions

Add support for stac/clac instructions to manipulate the flag
that controls the behaviour of Intel's Supervisor Mode Access
Prevention (SMAP) feature.

Tested by:	dim
Obtained from:	OpenBSD