Fix the asmprinter so that a globalvalue can specify an explicit alignment

Fix the asmprinter so that a globalvalue can specify an explicit alignment
smaller than the preferred alignment, but so that the target can actually
specify a minimum alignment if needed.  This fixes some objc protocol
failures Devang tracked down.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37373 91177308-0d34-0410-b5e6-96231b3b80d8