RFR: 8197981: Missing return statement in __sync_val_compare_and_swap_8

[Aleksey Shipilev]

This is similar to JDK-8254144 (that only did this for `os_linux_zero.cpp`), but a fuller version that also does
`os_bsd_zero.cpp`. The patch is already in 7u, and needs forward porting. It takes the shape of JDK-8254144, that is
with the comment: https://github.com/openjdk/jdk/commit/8f9e4792

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Commit messages:
 - Fix the comment in os_linux_zero too
 - 8197981: Missing return statement in __sync_val_compare_and_swap_8

Changes: https://git.openjdk.java.net/jdk/pull/739/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=739&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8197981
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/739.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/739/head:pull/739

PR: https://git.openjdk.java.net/jdk/pull/739