[Sparc] Add support for 13-bit PIC

Summary: When compiling with -fpic, in contrast to -fPIC, use only the
immediate field to index into the GOT. This saves space if the GOT is
known to be small. The linker will warn if the GOT is too large for
this method.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

Subscribers: brad, fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D47136

llvm-svn: 334383