Darwin builds aren't using the correct suffix for shared libraries or
proc macros (.dylib). This fixes that.
Bug: 155302034
Test: "cd external/rust; mma" no longer indicates wrong extension.
Change-Id: I5093e0bcc78dbb2dd9fcf39069f326101bb8f411
Support for building Rust modules on darwin hosts.
Bug: 140640858
Test: m libremain works on darwin
Change-Id: Ieb1ff9167de34cffbebebab31fa48da07081c8a6