ArmPkg: move RVCT PLATFORM_FLAGS override into ArmHvcLib/ArmSmcLib

ArmPkg: move RVCT PLATFORM_FLAGS override into ArmHvcLib/ArmSmcLib

Currently, each ARM platform built with RVCT that uses ArmHvcLib
or ArmSmcLib needs to specify a CPU target that implements both the
security and virtualization extensions, so that the assembler does
not choke on the 'hvc' and 'smc' instructions in ArmHvcLib/ArmSvcLib.
Let's move these overrides into the module .INFs so we can lift this
requirement at the platform side.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>