builtins: correct function name for AEABI

If `COMPILER_RT_ARMHF_TARGET` is set , the definition of the AEABI runtime
function `__aeabi_fcmpun` is misspelt: `__aeabi_fcmpum` instead of
`__aeabi_fcmpun`.

Patch by Konstantin Schwarz!

llvm-svn: 362424