ArmPkg/GicV3: use GICv3 generic sysreg names only for GNU as

ArmPkg/GicV3: use GICv3 generic sysreg names only for GNU as

The GNU assembler extends the generic notation for IMPLEMENTATION
DEFINED system registers to support any system register, so that
system registers defined by newer versions of the architecture can
still be used by older versions of the toolchain.

Clang before v3.6 supports the generic notation, but does not
support this extension, nor does it need to in the particular case
of the GICv3 support code, since it knows the GICv3 registers by
their architectural names. So only redefine their real names to
their generic aliases if we are not using clang.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18192 6f19259b-4bc3-4df7-8a09-765794883524