ArmPlatformPkg/ArmPlatformLib: Added support for ArmPlatformIsPrimaryCore()

ArmPlatformPkg/ArmPlatformLib: Added support for ArmPlatformIsPrimaryCore()

Checking if a core if the primary/boot core used to be done with the macro
IS_PRIMARY_CORE().
Some platforms exposes configuration registers to change the primary core.
Replacing the macro IS_PRIMARY_CORE() by ArmPlatformIsPrimaryCore() allows
some flexibility in the way to check the primary core.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Ryan Harkin <ryan.harkin@linaro.org>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14344 6f19259b-4bc3-4df7-8a09-765794883524