[WebAssembly] Fix libcall signature lookup

RuntimeLibcallSignatures previously manually initialized all the libcall
names into an array and searched it linearly for the first match to lookup
the corresponding index.
r322802 switched that to initializing a map keyed by the libcall name.
Neither of these approaches works correctly because some libcall numbers use
the same name on different platforms (e.g. the "l" suffixed functions
use f80 or f128 or ppcf128).

This change fixes that by ensuring that each name only goes into the map
once. It also adds tests.

Differential Revision: https://reviews.llvm.org/D42271

llvm-svn: 322971