RFR: 8294197: Zero: JVM_handle_linux_signal should not assume deopt NOPs

[Aleksey Shipilev]

This is a simple regression from Loom added code. In `JVM_handle_linux_signal` there is a new block that handles deopt NOPs. Zero does not have them, and so it currently encounters `ShouldNotReachHere()` when entering the block and doing either `os::Posix::ucontext_get_pc(uc)` or `NativeDeoptInstruction::is_deopt_at(pc)`.

This block should not be entered with Zero at all. 

Additional testing:
 - [x] Artificially crashing Linux x86_64 Zero fastdebug now produces a sensible hs_err

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Commit messages:
 - Argh.
 - Add end clause comment
 - Fix

Changes: https://git.openjdk.org/jdk/pull/10389/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10389&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294197
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10389.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10389/head:pull/10389

PR: https://git.openjdk.org/jdk/pull/10389