kbuild: speed up checksyscalls.sh

checksyscalls.sh is run at every "make" run while building the kernel,
even if no files have changed. I looked at where we spend time in
a trivial empty rebuild and found checksyscalls.sh to be a source
of noticeable overhead, as it spawns a lot of child processes just
to call 'cat' copying from stdin to stdout, once for each of the
over 400 x86 syscalls.

Using a shell-builtin (echo) instead of the external command gives
us a 13x speedup:

    Before		   After
real	0m1.018s       real	0m0.077s
user	0m0.068s       user	0m0.048s
sys	0m0.156s       sys	0m0.024s

The time it took to rebuild a single file on my machine dropped
from 5.5 seconds to 4.5 seconds.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>