ArmPlatformPkg/ArmPlatformStackLib: switch to ASM_FUNC() asm macro

ArmPlatformPkg/ArmPlatformStackLib: switch to ASM_FUNC() asm macro

Annotate functions with ASM_FUNC() so that they are emitted into
separate sections.

While we're at it, replace some inefficient uses of LoadConstantToReg(),
and remove the workaround that was added to allow conditional branches
to functions with external linkage.

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