swift234

Provide muloti4 on all archs, not just arm64.

Although this builtin is provided by clang rt builtins,
it isn't provided by libgcc, which is the default
runtime library on Linux, even when compiling with clang.
This implementation is copied here to avoid a new dependency
on compiler-rt on Linux.

Open up the ifdef to include non-Apple (i.e. Linux) builds.

rdar://problem/22616842

Swift SVN r31848