stand: Use -Oz/-Os for all loader/stand builds.

While we're not super size constrained, the x86 BIOS /boot/loader has
to be less than about 520k-530k to be reliable. The LUA loader is at
this size today. -Oz saves 15-20% on the size, keeping us safely small
enough (comparable to where we were with the 4th loader). This will
also help with sjg's work on bringing in bearssl, though we may again
be looking for space in the LUA loader.

Size table for clang 6.0.0:
default -O1 -Os -Oz
4th 442368 417792 389120 376832
lua 524288 479232 446464 430080

Tested by: kevans91@ (ubldr on armv7), dhw@ (loader on amdy64)
Differential Revision: https://reviews.freebsd.org/D16724