Fix assembler errors in generic arm strlen.c.

Tested using a static version of the strlen libc_test program
on a nexus7 that uses the generic code.

Merge from internal master.

(cherry-picked from d8d10a8994472e40d19301b7087806630877b4d5)

Change-Id: I88f7dc01dc5b5c3ac2d5580d92153bc1bc36c564