[compiler-rt] Add back ARM EABI aliases where legal.

r303188 removed all the uses of aliases for EABI functions from
compiler-rt, because some of them had mismatched calling conventions.
Obviously, we can't use aliases for functions which don't have the same
calling convention, but that's only an issue for floating-point
functions with the hardfloat ABI.  In other cases, the stubs increase
size and reduce performance for no benefit.

This patch adds back the aliases, with appropriate checks to make sure
they're only used in cases where the calling convention matches.

llvm-svn: 314851