Implement cfg(target_abi) (RFC 2992)

Add an `abi` field to `TargetOptions`, defaulting to "". Support using
`cfg(target_abi = "...")` for conditional compilation on that field.

Gated by `feature(cfg_target_abi)`.

Add a test for `target_abi`, and a test for the feature gate.

Add `target_abi` to tidy as a platform-specific cfg.

This does not add an abi to any existing target.

src/test/ui/cfg/cfg-target-abi.rs

@@ -0,0 +1,10 @@
+// run-pass
+#![feature(cfg_target_abi)]
+
+#[cfg(target_abi = "eabihf")]
+pub fn main() {
+}
+
+#[cfg(not(target_abi = "eabihf"))]
+pub fn main() {
+}