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

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.

Reviewed by:	kib, arichardson
MFC after:	1 month
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D23362

sys/conf/Makefile.mips

@@ -50,12 +50,9 @@ ARCH_FLAGS+=-mabi=${MIPS_ABI}
 EXTRA_FLAGS=-fno-pic -mno-abicalls -G0 -DKERNLOADADDR=${KERNLOADADDR}
 EXTRA_FLAGS+=-${MIPS_ENDIAN}
 
-HACK_EXTRA_FLAGS=-shared
-
 # We add the -fno-pic flag to kernels because otherwise performance
 # is extremely poor, as well as -mno-abicalls to force no ABI usage.
 CFLAGS+=${EXTRA_FLAGS} $(ARCH_FLAGS)
-HACK_EXTRA_FLAGS+=${EXTRA_FLAGS} $(ARCH_FLAGS)
 TRAMP_ARCH_FLAGS?=$(ARCH_FLAGS)
 TRAMP_EXTRA_FLAGS=${EXTRA_FLAGS} ${TRAMP_ARCH_FLAGS}
 # Kernel code is always compiled with soft-float on MIPS

sys/conf/Makefile.powerpc

@@ -39,7 +39,6 @@ INCLUDES+= -I$S/contrib/libfdt
 # Force __SPE__, since the builtin will be removed later with -mno-spe
 CFLAGS.gcc+= -mabi=spe -D__SPE__
 CFLAGS.clang+= -mspe -D__SPE__ -m32
-HACK_EXTRA_FLAGS= -shared -m32 -mspe -D__SPE__
 .endif
 CFLAGS+= -msoft-float
 CFLAGS.gcc+= -Wa,-many

sys/conf/kern.post.mk

@@ -226,10 +226,9 @@ kernel-clean:
 # This is a hack.  BFD "optimizes" away dynamic mode if there are no
 # dynamic references.  We could probably do a '-Bforcedynamic' mode like
 # in the a.out ld.  For now, this works.
-HACK_EXTRA_FLAGS?= -shared
 hack.pico: Makefile
 	:> hack.c
-	${CC} ${HACK_EXTRA_FLAGS} -nostdlib hack.c -o hack.pico
+	${CC} -shared ${CFLAGS} -nostdlib hack.c -o hack.pico
 	rm -f hack.c
 
 offset.inc: $S/kern/genoffset.sh genoffset.o