missing asm/cachectl.h header

[Matthias Klose <doko@cs.tu-berlin.de>]

currently gcc-4.3 fails to build on m68k:

libtool: compile:  /build/buildd/gcc-4.3-4.3-20080112/build/./gcc/xgcc -B/build/buildd/gcc-4.3-4.3-20080112/build/./gcc/ -B/usr/m68k-linux-gnu/bin/ -B/usr/m68k-linux-gnu/lib/ -isystem /usr/m68k-linux-gnu/include -isystem /usr/m68k-linux-gnu/sys-include -DHAVE_CONFIG_H -I. -I../../../src/libffi -I. -I. -I../../../src/libffi/include -Iinclude -I../../../src/libffi/src -Wall -g -fexceptions -O2 -g -g -O2 -MT src/m68k/ffi.lo -MD -MP -MF src/m68k/.deps/ffi.Tpo -c ../../../src/libffi/src/m68k/ffi.c  -fPIC -DPIC -o src/m68k/.libs/ffi.o
../../../src/libffi/src/m68k/ffi.c:13:26: error: asm/cachectl.h: No such file or directory
../../../src/libffi/src/m68k/ffi.c: In function 'ffi_prep_closure_loc':
../../../src/libffi/src/m68k/ffi.c:269: error: 'FLUSH_SCOPE_LINE' undeclared (first use in this function)
../../../src/libffi/src/m68k/ffi.c:269: error: (Each undeclared identifier is reported only once
../../../src/libffi/src/m68k/ffi.c:269: error: for each function it appears in.)
../../../src/libffi/src/m68k/ffi.c:270: error: 'FLUSH_CACHE_BOTH' undeclared (first use in this function)
make[6]: *** [src/m68k/ffi.lo] Error 1