This CL lands a workaround for a bug causing the linker to merge
ExternalOneByteStringGetChar() and ExternalTwoByteStringGetChar() which
leads to the generated vtable address checks failing on one of the
inputs.

To make the two function's machine code different (to prevent the
linker from merging them), this CL adds CHECKs of the arguments to both
functions.

Bug: chromium:1160961
Change-Id: Ifc4c6e4e05a394a6f27572877abb765d02fd23ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2640478Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72216}