Log Message:
gcc7: update to 7.3.0

Local changes:
Remove patch hunks merged upstream (non-c99 compatibility cabs workaround,
linking to libc by default on netbsd).

GCC changes:
Added code generation options to mitigate Spectre variant 2 (CVE-2017-5715)
for the powerpc and x86 targets.

RTEMS: Support has been added for Epiphany target.

List of bugs fixed:
https://gcc.gnu.org/bugzilla/buglist.cgi?bug_status=RESOLVED&list_id=199567&resolution=FIXED&target_milestone=7.3