toolchain/gcc: Add "memory" clobber to asm traps before CODE_UNREACHABLE

GCC 14.3 will happily delete any code that appears before
__builtin_unreachable that isn't separated with an obvious branch. That
includes __asm__ statements, even those which generate traps.

The failure case that I debugged was on x86 in
z_check_stack_sentinel. There is a store to restore the sentinel to the
correct value just before the ARCH_EXCEPT, and that macro emits 'int $32'
followed by CODE_UNREACHABLE. Because the compiler didn't understand that
ARCH_EXCEPT was changing execution flow, it decided that the sentinel
restoring store 'couldn't' be reached and elided it.

I added the "memory" clobber to the asm statement in ARCH_EXCEPT before
CODE_UNREACHABLE to enforce that all pending store operations be performed
before the asm statement occurs. This ensures that they are not deleted by
the compiler.

I think this might be a GCC bug. The GCC documentation explicitly documents
that asm statements which change the flow of control should be followed by
__builtin_unreachable.

Signed-off-by: Keith Packard <keithp@keithp.com>