This adds initial support for concurrently patching jump table slots. It
is needed once different Isolates share code (for the --wasm-shared-code
feature). We need to ensure that instructions holding the target address
within a jump table slot do not cross cache-line boundaries. To do this,
the jump table has been split into consecutive pages.

Note that this also adds a stress test for multiple threads hammering at
a single slot concurrently. The test is currently limited to the ia32
and the x64 architecture, but will be extended to cover others. The test
reliably triggers tearing of the target address on almost every run of
the test and hence serves to prevent regressions.

R=clemensh@chromium.org
TEST=cctest/test-jump-table-assembler
BUG=v8:8018

Change-Id: Ife56bbb61ffcae5d8906ca7b8c604b195603707c
Reviewed-on: https://chromium-review.googlesource.com/1163664
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54942}