RFR: 8256831: MIPS Zero builds fail with undefined __atomic_compare_exchange_8

[Aleksey Shipilev]

If you try to build `linux-mipsel-zero-fastdebug`, this happens:







I think it relates to [JDK-8253970](https://bugs.openjdk.java.net/browse/JDK-8253970) that introduced `atomic_compare_exchange` on those paths, but maybe the issue exists for longer. 

Various other reports for build failures like this suggest the binary should link with `libatomic`. GCC's `libatomic` is the library that provides runtime support for atomics not supported in hardware, which seems to include 8-byte CAS for MIPS, but not any other OpenJDK platform.

Attention @DamonFool, who must be running into this problem for their MIPS builds?

Testing:
 - [x] Linux mipsel Zero fastdebug build (together with JDK-8256829 fix)

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Commit messages:
 - 8256831: MIPS Zero builds fail with undefined __atomic_compare_exchange_8

Changes: https://git.openjdk.java.net/jdk/pull/1375/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1375&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256831
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1375.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1375/head:pull/1375

PR: https://git.openjdk.java.net/jdk/pull/1375