Don't ask libc for PT feature w/o kernel support.

Check that the kernel supports the Tagged Pointers feature before asking
libc to disable/enable pointer tagging.

Bug: 153114333
Test: Build coral, note no messages from SetHeapTaggingLevel.
Test: atest CtsTaggingHostTestCases

Change-Id: Id5fbd731aa70f7b0a75525387479f6123628a012
Merged-In: Id5fbd731aa70f7b0a75525387479f6123628a012