Improve *BSD native ABI detection

[LemonBoy]

At the moment zig detects the native ABI as .gnu, that may cause some problems as many *BSD libcs are not fully gnu-compliant (eg. NetBSD has no sincos, a GNU extension, but since we're passing LLVM the wrong triple we end up with some missing symbols).

cc @mikdusan as I guess you'd love some BSD action

[mikdusan]

• the following compares -triple for both zig cc and system clang for compiling empty.c on a freebsd host

"/usr/home/mike/project/zig/work/main/_build/zig" -cc1 -triple x86_64-unknown-freebsd-gnu
"/usr/bin/clang" -cc1 -triple x86_64-unknown-freebsd13.0

zig and llvm model things slightly differently:

• Target.abi.none vs (Triple.Environment.unknown and .hasEnvironment())
• currently when Target.abi == none we omit the Environment string component of -triple and it ends up as hasEnvironment() == false

Along those lines we could simply do this:

• set .abi = .none when detecting target on *bsd hosts (and darwin)
• bad: this lies to the reader because we in fact have an ABI

But don't we actually want to capture the idea that .abi is optional? We could either do that through using zig optional but it would have higher impact on src/*.zig codebase, or we could add enum .unspecified or similar.

Thoughts?

[LemonBoy]

set .abi = .none when detecting target on *bsd hosts (and darwin)

When targeting ARM you still want to specify EABI/EABIHF as ABI, at least for *BSD targets.

or we could add enum .unspecified or similar.

On the LLVM side .none doesn't really mean "no ABI" but more "unspecified, do not make any assumption".