Adds support for 64-bit integer and some float operations for 32-bit x86.

On i386 platforms, modern compilers are emitting call-outs to 64-bit integer
arithmetic. This patch adds support for these functions. Implementations are
taken from LLVM's compiler-rt library, which appears to have a compatible
license with openlibm. Appropriate license information has been added to
LICENSE.md to reflect this merge.