Compile hack.c with normal CFLAGS + -shared -nostdlib.

[jhb]

Summary

Originally, hack.c was compiled into a shard object with just -shared
-nostdlib. This assumed that ${CC} did not require any additional
flags for ABIs, cross-building, etc.

When kern.post.mk was created in r89509 by reducing duplication in
kernel Makefile.<arch> files, the -shared flag was moved into a
HACK_EXTRA_FLAGS variable so that sparc64 could override it with
-Wl,-shared. The sparc64 hack was removed in r111650, but
HACK_EXTRA_FLAGS was left in place. Over time, we have started
support toolchains that require flags to support alternate ABIs on
MIPS and PowerPC and started (ab)using HACK_EXTRA_FLAGS to set only
those flags.

I need to fix risc-v to pass -mno-relax to the hack.c build for lld in
llvm 10, and the patches to support cross-build from non-FreeBSD hosts
need to include -target for clang in CFLAGS for hack.c. Rather than
adding more hacks into HACK_EXTRA_FLAGS, just use the full set of
CFLAGS with hack.c.

Test Plan

• make tinderbox
• CheriBSD has used a similar-ish change (it copied the expresion from genoffsets.c to strip lto and one other flag, but I don't think those matter for the empty hack.c case)

[kib]

BTW I think that pico extension is not correct (as much as extension can be).

[arichardson]

Yes I agree, removing the LTO option is not necessary.
I think in https://github.com/CTSRD-CHERI/cheribsd/commit/8041a7e459d1d6e453d082ff57db2f3f94ca1a3c I just copied from the genassym.o line.

[jhb]

It used to be .So but was changed to .pico in https://svnweb.freebsd.org/base?view=revision&revision=306297. In this particular case there isn't a .so that it would collide with, so it could go back to .So perhaps, but I'm not going to bother with that anytime soon.