ArmPkg RVCT: add asm macro combining EXPORT, AREA and label definition

ArmPkg RVCT: add asm macro combining EXPORT, AREA and label definition

In response to Leif's request earlier, this adds a new RVCT assembler
macro to centralize the exporting of assembly functions including the
EXPORT directive (so the linker can see it), the AREA directive (so
it's in its own section for code size reasons) and the function label
itself.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eugene Cohen <eugene@hp.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19098 6f19259b-4bc3-4df7-8a09-765794883524