[orc][cmake] Check if 8 byte atomics require libatomic for unittest

rL319838 introduced SymbolStringPool which uses 8 byte atomics for
reference counters. On systems which do not support such atomics
natively such as MIPS32, explicitly add libatomic as one of the
libraries for SymbolStringPool's unittest.

Reviewers: lhames, beanz

Differential Revision: https://reviews.llvm.org/D41010

llvm-svn: 321225