Fix this build for clang.
MFC r259730:
To avoid having to explicitly test COMPILER_TYPE for setting
clang-specific or gcc-specific flags, introduce the following new
variables for use in Makefiles:
CFLAGS.clang
CFLAGS.gcc
CXXFLAGS.clang
CXXFLAGS.gcc
In bsd.sys.mk, these get appended to the regular CFLAGS or CXXFLAGS for
the right compiler.
MFC r259913:
For libstand and sys/boot, split off gcc-only flags into CFLAGS.gcc.
MFC r259927:
Fix pc98 build, by also forcing COMPILER_TYPE in sys/boot/pc98/boot2's
Makefile.
Pointy hat to: dim
5fce04bd49