Allow memset and memrchr to be ifuncs.

We don't currently have any, but Qualcomm is looking at a custom memset
for Oryon, and it's more uniform to just treat all these functions the
same.

Bug: https://issuetracker.google.com/330105715
Test: treehugger
Change-Id: I84f589c0b06d5477c9b59640970fdc9196abb543
1 file changed