[AArch64] Add support for dllimport of values and functions

Previously, the dllimport attribute did the right thing in terms
of treating it as a pointer to a value, but this makes sure the
names get mangled properly, and calls to such functions load the
function from the __imp_ pointer.

This is based on SVN r212431 and r212430 where the same was
implemented for ARM.

Differential Revision: https://reviews.llvm.org/D38530

llvm-svn: 316555