RFR: 8254611: x86_32: Call to IRT::at_unwind clobbers rthread after JDK-8253180

[Aleksey Shipilev]

There are massive x86_32 tier1 failures, bisection points to JDK-8253180. I think I know why this happens: in
`InterpreterMacroAssembler::remove_activation`, there is a new call_VM to `InterpreterRuntime::at_unwind`, which broke
the `rthread` (`rcx`) that x86_32 needs later. x86_64 is not affected, because it carries it in `r15_thread`.

Attention @fisk.

Additional testing:
  - [ ] x86_32 tier1

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

Commit messages:
 - 8254611: x86_32: Call to IRT::at_unwind clobbers rthread after JDK-8253180

Changes: https://git.openjdk.java.net/jdk/pull/615/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=615&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254611
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/615.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/615/head:pull/615

PR: https://git.openjdk.java.net/jdk/pull/615