macOS 'universal' binary support

[vickenty]

From #2 (comment)

Perl interpreters shipped with macOS Sierra (and possibly others) are built as universal binaries, and the same applies to XS extensions.

Since we build C glue using the same options, C compiler produces a static library which is also a universal binary. rustc is currently not able to link to such libraries:

warning: warning: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: archive library: /Users/daniel/code/perl-xs/t/target/release/build/perl-sys-4230308055e1a1c1/out/libperlsys.a will be fat and ar(1) will not be able to operate on it
error: failed to add native library /Users/daniel/code/perl-xs/t/target/release/build/perl-sys-4230308055e1a1c1/out/libperlsys.a: File too small to be an archive