The GNU profiling support indicates that the interface is _mcount rather than mcount on GLIBC Linux for AArch64.
I realized after posting this I made a mistake in compilation and test. Because of this clang wasn't properly tested. This patch actually causes "ninja check-all" to fail for non-GNU AArch64 Linux targets.
I am able to link with clang using -pg -meabi gnu src.c
I'd like to withdraw this patch from revew.