MdePkg Base.h: Always define ASM_PFX

MdePkg Base.h: Always define ASM_PFX

Some compilers may define __USER_LABEL_PREFIX__ to determine the
prefix used with ASM_PFX. Otherwise, IA32 will use a single underscore
'_' character, and all other architectures will use an empty prefix.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16019 6f19259b-4bc3-4df7-8a09-765794883524