MdePkg/BaseLib: Replaced inline assembly for ARMGCC by GCC assembly source file

MdePkg/BaseLib: Replaced inline assembly for ARMGCC by GCC assembly source file

The registers used to passed arguments to the DxeCore entrypoint in the inline
assembly function InternalSwitchStackAsm() where switched at compilation time.
This change replaces the inline assembly by a pure assembly source file to
avoid this kind of optimization.

Note: Inline assembly is generally not recommanded in system programming.

Reviewed-by: lgao4
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12674 6f19259b-4bc3-4df7-8a09-765794883524