Adds handling of a signal interrupting the ibl hit path's jump, where the target tag is not conveniently sitting in a register like it is in the rest of the ibl. We have to instead look at the jump's cache target and find the fragment to unlink from there.
Also fixes up the i#2328 support for a signal interrupting clean_call_{save,restore} to work for AArch64.
Adds a new test, client.cleancallsig, which tries to trigger both of these cases: the client adds a lot of clean calls while the app runs a loop with a lot of indirect branches along with an itimer. I managed to hit each of the new features at least once running this test in a loop on each platform except the ibl jump on ARM and AArch64. I left an ASSERT_NOT_TESTED for those.
Issue: #2328 (closed) Fixes #2042 (closed)