RFR: 8294594: Fix cast-function-type warning in signal handling code

[Aleksey Shipilev]

After [JDK-8294314](https://bugs.openjdk.org/browse/JDK-8294314), we would have signals_posix.cpp excluded with cast-function-type warning:

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/10494/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10494&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294594
  Stats: 8 lines in 3 files changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10494.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10494/head:pull/10494

PR: https://git.openjdk.org/jdk/pull/10494