mips: syscalls: define syscall offsets directly in <asm/unistd.h>

There is no good reason to generate the syscall offset macros by
scripting since they are not derived from the syscall tables.

Define __NR_*_Linux macros directly in arch/mips/include/asm/unistd.h,
and clean up the Makefile and the shell script.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>