pc64/acpica: specify cmpw in acpi_wakecode.S

The "cmp" instruction is ambiguious in acpi_wakecode.S.  The GNU assembler
should emit an error, but instead it just assumes that the operand is of
a wide type.  The clang assembler does throw an error, so let's explicitly
use "cmpw" to resolve the ambuiguity.
boot/dloader: Remove duplicate CFLAGS

The -ffreestanding flag gets defined by ../pc32/Makefile.inc which
both platforms pull in.  The -m32 flag is defined by ../pc64/Makefile
which only x86_64 pulls in.
boot/pc32: specify some flags based on CCVER

Some flags are gcc-specific, so only set them if CCVER begins with
the substring "gcc".  Some assembly files can only be built by the
binutils GNU assembler, so define ASSEMBLER flags which CCVER starting
with "clang" will augment.  There is no change for the current base
compilers.
libstand: Isolate gcc-specific flags, avoid native compilations

Only set -mpreferred-stack-boundary when using GCC.  While here, add
a bunch of flags that avoid cpu-specific instructions.  Similar flags
are seen in the boot code.
kern.mk: isolate gcc-only flags to gcc compilers (determined by CCVER)
dev/drm: For now, only build radeon drivers with base compilers

Currently, at least clang35 can't build radeon drivers due to how system
headers are included.  I suspect dports gcc compilers may also have the
same issue.  Until this issue is understood and fixed, limit the building
of radeon modules to the base compilers.