MdePkg: BaseSynchronizationLib: fix Increment/Decrement retvals for ARM

MdePkg: BaseSynchronizationLib: fix Increment/Decrement retvals for ARM

The procedure call standard dictates that we move the result to r0 before
returning.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Tyler Smith <tylers@hp.com>
Reviewed-by: Eugene Cohen <eugene@hp.com>
Reviewed-by: Leif Lindholm <leif.lindholm@arm.com>
Reviewed-by: Ronald Cron <Ronald.Cron@arm.com>
[lersek@redhat.com: cleaned up commit message]

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17068 6f19259b-4bc3-4df7-8a09-765794883524