Make gcc accept -Oz as an alias for -Os for compatibility with clang.

With clang -Os doesn't generate the smallest code possible but some middle
ground between optimization for speed and optimization for size. A new -Oz
option was introduced for optmization for size only. We need that for our
floppies, otherwise they overflow. Making gcc accept -Oz too makes our life
easier.

ok millert@, deraadt@, robert@

gcc 4.9 makes unknown -Wno-* vanish.

Reimplement that from scratch in our ancient gcc, because it's really
useful for porting newer code and dealing with compiler variations.
(slightly tweaked to reset location to unknown location after the okays)

okay kettenis@ jasper@

found out https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28322
after the patch, which explains a similar reasoning better, and leads
to another patch for older GCC, possibly GPLv3.

Backport support for -Wframe-larger-than=N to base GCC

This is the flag name that modern GCC and Clang have de facto
standardized on for the functionality that we locally named
-Wstack-larger-than-N.

ok brad, miod

Add support for PIE-by-default in both ld and gcc. This is a completely
different approach than the one taken in kurt@'s original diff, but deemed
better after discussion and diff exchange with kettenis@ and matthew@.

Lots of feedback by kettenis@ and matthew@, prodding and encouragement by
deraadt@.

ok kettenis@ matthew@

turn off strict overflow for all optimizations on OpenBSD, also disable wrapv.
ok kettenis@ robert@, tested by marco@, kevlo@ myself

Don't include -ftree-vrp in -O2 and higher as it has known bugs in gcc 4.2.
Fixes jdk builds among other things.

ok robert@ espie@

merge more local changes:
- disable unsafe builtins
- create a stub for -Wbounded
- move strict aliasing to -O3

merge some gcc-local(1) changes:
- disable -fident by default
- disable -fstrict-aliasing with -O2
- add support for -Wstack-larger-than-N (only for i386 and amd64 for now)

branches: 1.1.1;
Initial revision