[libc++] Add hack to allow ubsan to work w/o compiler-rt (__muloti4 is undefined)

Summary:
Using int128_t with UBSAN causes link errors unless compiler-rt is providing the runtime library.
Specifically ubsan generates calls to __muloti4 but libgcc doesn't provide a definition.

In order to avoid this, and allow users to continue using sanitized versions of libc++, this patch introduces a hack.
It adds a cribbed version of  the compiler-rt builtin to the libc++ filesystem sources.

I don't think this approach will work in the long run, but it seems OK for now.

Also see:

https://bugs.llvm.org/show_bug.cgi?id=30643
https://bugs.llvm.org/show_bug.cgi?id=16404


Reviewers: mclow.lists, ldionne, rsmith, jyknight, echristo

Reviewed By: echristo

Subscribers: dberris, cfe-commits

Differential Revision: https://reviews.llvm.org/D49828

llvm-svn: 337990