AArch64: Fix art_quick_string_compareto.

Though __memcmp16() is implemented in the same module as the caller, it
is still possible that the toolchain would put __memcmp16() into plt. In
that case, IP registers can be trashed when loading the function address.
Use x14/x15 to replace IP0/IP1.

Bug: 16974467

(cherry picked from commit 62ddb328)

Change-Id: I40e39d075860bc78624ce6ef8b4f8e33e57fc58c