MdePkg RVCT: add definition of UNREACHABLE

MdePkg RVCT: add definition of UNREACHABLE

The RVCT compiler in --gnu mode appears to simply strip of the __builtin
prefix when it encounters calls to __builtin_xxx() functions, and so
the __builtin_unreachable() we emit for GCC results in linker errors
regarding undefined references against 'unreachable()'.

So define UNREACHABLE() to a NOP instead.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>