ArmPkg RVCT: add ADRL/LDRL macro equivalents

ArmPkg RVCT: add ADRL/LDRL macro equivalents

The GCC ARM builds have access to ADRL/LDRL macros that emit relative
symbol references, i.e., references that do not require fixing up at
load time (or FV generation time for XIP modules)

Implement equivalent functionality for RVCT: note that this does not
use movw/movt pairs, but the more compatible add/add/add or add/add/ldr
sequences (which Clang does not support, unfortunately, hence the use
of movw/movt for the GCC toolchain family)

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>