RFR: 8254362: x86_32 builds fail after JDK-8253180

[Aleksey Shipilev]

`r15_thread` is not available on x86_32. I noticed that JDK-8253180 introduces:

void C1SafepointPollStub::emit_code(LIR_Assembler* ce) {
#ifdef _LP64
...
  __ movptr(Address(r15_thread, JavaThread::saved_exception_pc_offset()), rscratch1);
...
#else
  ShouldNotReachHere();
#endif /* _LP64 */
}

...and we should do the same in `C2SafepointPollStubTable` to unbreak x86_32.

Testing:
 - [x] x86_32 build
 - [x] x86_32 hotspot:tier1 (lots of unrelated failures that look like JDK-8254125, never a `ShouldNotReachHere`)
 - [x] x86_64 build
 - [x] x86_64 hotspot:tier1

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

Commit messages:
 - 8254362: x86_32 builds fail after JDK-8253180

Changes: https://git.openjdk.java.net/jdk/pull/593/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=593&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254362
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/593.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/593/head:pull/593

PR: https://git.openjdk.java.net/jdk/pull/593