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

There are massive x86_32 failures in tier1, most often taking the form of NPE with "There cannot be a NullPointerException".

$ CONF=linux-x86-server-fastdebug make clean images run-test TEST=hotspot:tier1

TEST: compiler/c2/Test6910605_2.java
TEST JDK: /home/shade/trunks/jdk/build/linux-x86-server-fastdebug/images/jdk

...
Caused by: java.lang.NullPointerException: There cannot be a NullPointerException at bci 19 of method int jdk.internal.org.objectweb.asm.SymbolTable.hash(int, java.lang.String, java.lang.String, java.lang.String)

Bisection points to JDK-8253180.

This also reproduces only intermittently, when a lot of tests are running. Re-running the tests individually make them pass.