ArmVirtPkg: clean up assembly source files

ArmVirtPkg: clean up assembly source files

This updates all assembly source files under ArmVirtPkg to mark
exported functions as ASM_FUNC(), which puts them in a separate
section, allowing the linker to prune code that is left unused.

At the same time, clean up the code to get rid of LoadConstantToReg()
instances involving symbol references, each of which emits an absolute
literal, and hence and entry in the PE/COFF .reloc table.

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