ArmPlatformPkg/PrePiHobListPointerLib: use thread ID register

ArmPlatformPkg/PrePiHobListPointerLib: use thread ID register

This updates the PrePiHobListPointerLib implementation in ArmPlatformPkg
to move away from ArmPlatformGlobalVariableLib and instead use the thread
ID CPU registers (TPIDRURW and TPIDR_EL0 for v7 and v8, respectively)
for storing the HobList pointer.

Since PrePiHobListPointerLib is specific to PrePi (where PEI core is skipped)
we can share these registers with the PEI services table pointer. By the
same reasoning, the PEI services table pointer and the HobList pointer
already shared the same offset in the ArmPlatformGlobalVariable array.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18982 6f19259b-4bc3-4df7-8a09-765794883524