[ARM] Add mapping symbols to PLT entries

Make PLT entry atoms represent mapping symbols in the Release mode,
while in the Debug mode they are still function-like symbols
with regular names.
It's legal that mapping symbols denote unnamed parts of code,
and PLT entries are not required to have function-like names.

Differential Revision: http://reviews.llvm.org/D8819

llvm-svn: 234301