Add generic __bswap[ds]i2 implementations

Summary:
In FreeBSD we needed to add generic implementations for `__bswapdi2` and
`__bswapsi2`, since gcc 6.x for mips is emitting calls to these.  See:

https://reviews.freebsd.org/D10838 and https://reviews.freebsd.org/rS318601

The actual mips code generated for these generic C versions is pretty
OK, as can be seen in the (FreeBSD) review.

I checked over gcc sources, and it seems that it can emit these calls on
more architectures, so maybe it's best to simply always add them to the
compiler-rt builtins library.

Reviewers: howard.hinnant, compnerd, petarj, emaste

Reviewed By: compnerd, emaste

Subscribers: mgorny, llvm-commits, arichardson

Differential Revision: https://reviews.llvm.org/D33516

llvm-svn: 303866