ArmPlatformPkg/Sec: fix return_from_exception code and comment

The return_from_exception implementation in Sec/Helper.S (the GCC
version) deviates from the RVCT version, in a way that suggests that
both may have been broken at some point, and that they weren't fixed
in the same way nor at the same time.

So bring the GCC version in line with the RVCT version, and at the
same time, deobfuscate the comment.

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@19244 6f19259b-4bc3-4df7-8a09-765794883524