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
diff --git a/Android.bp b/Android.bp
index 62e26e7..905c682 100644
--- a/Android.bp
+++ b/Android.bp
@@ -143,8 +143,6 @@
             ],
             asflags: [
                 "-march=armv8-a+sve",
-                "-D__memset_aarch64=memset",
-                "-D__memrchr_aarch64=memrchr",
             ]
         },
     },