MFH: r459981

Update to the GCC 7.3 release.

This fixes 99+ bugs and includes code generation options to mitigate
Spectre Variant 2 (CVE 2017-5715) for the x86 and powerpc targets
(-mindirect-branch=, -mfunction-return=, and -mindirect-branch-register).

files/patch-armv7-support and files/patch-freebsd-unwind.h, which I
had backported are now included, so remove them.

Approved by: ports-secteam@ (swills)