core/cortex-m: Disable warning in vecttable.c when building with clang
core/cortex-m: Disable warning in vecttable.c when building with clang

The vecttable logic explicitly routes unused IRQs to IRQ_UNUSED_OFFSET,
and then assigns it to "null" at the very end. The result is something
like:

 [8] = __attribute__((used, weak, alias("default_handler")))
       irq_55_handler(void);
 [8] = __attribute__((used, weak, alias("default_handler")))
       irq_56_handler(void);
 ...
 [8] = (void*)0

This is intentional so that it works with a varying values for
CONFIG_IRQ_COUNT.

BRANCH=none
BUG=b:172020503
TEST=make buildall -j

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I3ba4eeaa46cd2c50c65c922f4c0c463ce1bb585e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3172038
Reviewed-by: Patrick Georgi <bc411205f21846a74924ee7b489c75617ec76078@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec