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


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@321225 91177308-0d34-0410-b5e6-96231b3b80d8

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