[POWERPC] Pass per-file CFLAGs for platform specific op codes

As a result of compiling all of the wrapper files for every platform
now, the kernel build can fail for toolchains that don't support various
op codes by default.  An example of this building a 7xx platform with
the ELD4.0 toolchain, is below:

/tmp/ccYjhJoL.s: Assembler messages:
/tmp/ccYjhJoL.s:42: Error: Unrecognized opcode: `mtdcr'
/tmp/ccYjhJoL.s:43: Error: Unrecognized opcode: `mfdcr'
make[1]: *** [arch/powerpc/boot/44x.o] Error 1

The following patch introduces additional CFLAGS for the 4xx specific
files and fixes the kernel compile.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>