gdb: Sync up x86-gcc-cpuid.h with cpuid.h from gcc 14 branch.

This is required for a later commit which requires "bit_SHSTK".

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Luis Machado <luis.machado@arm.com>