all_syscalls: use sed to extract defines from headers

all_syscalls: use sed to extract defines from headers

Posix-compliant awk does not seem capable of matching lines and
extracting capture groups of them.
Use sed instead.

Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/util-linux/051624b9256db27a731d62c031cb627d9f5a256e.camel@physik.fu-berlin.de/
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>
(cherry picked from commit 7e8bf34a2a54b639d95c51a4a0368b0c9578d612)