ArmPlatformPkg RVCT: drop dependency on GCC macro library

ArmPlatformPkg RVCT: drop dependency on GCC macro library

The RVCT .asm files include AsmMacroIoLib.h only for the definition of
LoadConstantToReg (), which makes it tedious to make change to that file
without the risk of making the RVCT assembler unhappy. So simply replace
LoadConstantToReg() with mov32, which does the right thing in all cases.

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