Skip guest register getting on arm32.

Currently, there is no support for a guest architecture when running
in an arm 32 bit process. So always return false in that case instead
of spamming the log with useless messages.

Bug: 384944467

Test: debuggerd tests pass with no extraneous error messages.
Change-Id: I8152f1e76031e8b765c4c594b0bbc6800ab60c9e