arch/x86/Makefile.inc: Pass $(AS) and $(CPP) to SeaBIOS

SeaBIOS’ Makefile requires cpp (C Preprocessor) to build. Modify
the xcompile script to search for cpp program path, and pass it to
SeaBIOS’ `Makefile.inc`. Also pass the program path for as (GNU assembler).

This is needed, so the crossgcc toolchain to build the SeaBIOS payload
under Mac OSX. OSX ships a cpp program, but it works differently
from GNU CPP, so we need to override it.

Change-Id: If996ffbb76ec4bd16079b54b41f3fac07bfe25be
Signed-off-by: Andrew Wu <arw@dmp.com.tw>
Reviewed-on: http://review.coreboot.org/3896
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Jonathan A. Kollasch <jakllsch@kollasch.net>

util/xcompile/xcompile

@@ -117,6 +117,7 @@ report_arch_toolchain() {
 	cat <<EOF
 # elf${TWIDTH}-${TBFDARCH} toolchain (${GCCPREFIX}gcc)
 CC_${TARCH}:=${GCCPREFIX}gcc ${CFLAGS}
+CPP_${TARCH}:=${GCCPREFIX}cpp
 AS_${TARCH}:=${GCCPREFIX}as ${ASFLAGS}
 LD_${TARCH}:=${GCCPREFIX}ld${LINKER_SUFFIX} ${LDFLAGS}
 NM_${TARCH}:=${GCCPREFIX}nm