builtin_compiler/build: Don't use *_FOR_BUILD when not cross compiling

builtin_compiler/build: Don't use *_FOR_BUILD when not cross compiling

Previously we were relying on CFLAGS_FOR_BUILD to be the same as CFLAGS
when not cross compiling, but this assumption didn't take into
consideration 32-bit builds on 64-bit systems. More generally, not
honoring CFLAGS is bad.

Automake is evidently too stupid to accept

if CROSS_COMPILING
CC = @CC_FOR_BUILD@
...
else
CC = @CC@
endif

without warning that CC has been already defined. The warnings are
harmless, but I'd prefer to avoid future reports about them, so define
proxy variables, which are assigned inside the conditional and then
unconditionally assigned to CC et al.

NOTE: This is a candidate for the 9.1 branch.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59737
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60038