sh: fix race in parallel out-of-tree build

Depending on which of the three dependencies for archprepare (in
arch/sh/Makefile) get built first, the directory include/asm-sh may or
may not exist when the maketools target is built. If the directory does
not exist, awk will fail to generate machtypes.h. This patch fixes this
by creating the directory before awk is executed.

Signed-off-by: Erik Johansson <erik.johansson@motorola.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>