[NVPTX] Do not emit .weak symbols for NVPTX

Summary:
".weak" symbols cannot be consumed by ptxas (PR21685). This patch makes the
weak directive in MCAsmPrinter customizable, and disables emitting ".weak"
symbols for NVPTX.

Test Plan: weak-linkage.ll

Reviewers: jholewinski

Reviewed By: jholewinski

Subscribers: majnemer, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D6455

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223077 91177308-0d34-0410-b5e6-96231b3b80d8