RFR: 8255741: Zero: print signal name in unhandled signal handler

[Aleksey Shipilev]

At least one test -- `runtime/Safepoint/TestAbortVMOnSafepointTimeout.java` -- fails on Zero with:

# Internal Error (/home/shade/trunks/jdk/src/hotspot/os_cpu/linux_zero/os_linux_zero.cpp:250), pid=560280, tid=560281
# fatal error: caught unhandled signal 4

The test expects "SIGKILL (sent by kill)" in the test output.

Additional testing:
 - [x] `TestAbortVMOnSafepointTimeout.java` now passes on Linux x86_64 Zero.

-------------

Commit messages:
 - 8255741: Zero: print signal name in unhandled signal handler

Changes: https://git.openjdk.java.net/jdk/pull/1001/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1001&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255741
  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1001.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1001/head:pull/1001

PR: https://git.openjdk.java.net/jdk/pull/1001