BaseTools/tools_def AARCH64: mark register x18 as reserved

BaseTools/tools_def AARCH64: mark register x18 as reserved

The AArch64 ABI classifies register x18 as a platform register, which
means it should not be used unless the code is guaranteed to run on a
platform that doesn't use it in such a capacity.

GCC does not honour this requirement by default, and so we need to tell
it not to touch it explicitly, by passing the -ffixed-x18 command line
option.

Link: https://bugzilla.tianocore.org/show_bug.cgi?id=625
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>