RFR(XS): 8049441: PPC64: Don't use StubCodeMarks for zero-length stubs

[Volker Simonis]

Hi,

could somebody please review and push the following small, PPC64-only
change to any of the hs team repositories:

http://cr.openjdk.java.net/~simonis/webrevs/8049441/
https://bugs.openjdk.java.net/browse/JDK-8049441

Background:

For some stubs we actually do not really generate code on PPC64 but
instead we use a native C-function with inline-assembly. If the
generators of these stubs contain a StubCodeMark, they will trigger
JvmtiExport::post_dynamic_code_generated_internal events with a zero
length code size. These events may fool clients like Oprofile which
register for these events (thanks to Maynard Johnson who reported this
- see http://mail.openjdk.java.net/pipermail/ppc-aix-port-dev/2014-June/002032.html).

This change simply removes the StubCodeMark from
ICacheStubGenerator::generate_icache_flush() and generate_verify_oop()
because they don't generate assembly code. It also removes the
StubCodeMark from generate_throw_exception() because it doesn't really
generate a plain stub but a runtime stub for which the JVMT dynamic
code event is already generated by RuntimeStub::new_runtime_stub() ->
CodeBlob::trace_new_stub() ->
JvmtiExport::post_dynamic_code_generated().

Thank you and best regards,
Volker