x86/syscalls: Switch to generic syscallhdr.sh

Many architectures duplicate similar shell scripts.

Converts x86 to use scripts/syscallhdr.sh.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210517073815.97426-7-masahiroy@kernel.org