Remove the DW_AT_MIPS_linkage name attribute when we don't need it

output (we're emitting a specification already and the information
isn't changing) and we're not in old gdb compat mode.

Saves 1% on the debug information for a build of llvm.

Fixes rdar://11043421

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162493 91177308-0d34-0410-b5e6-96231b3b80d8