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.

compiler/rustc_span/src/symbol.rs

@@ -353,6 +353,7 @@ symbols! {
         cfg_eval,
         cfg_panic,
         cfg_sanitize,
+        cfg_target_abi,
         cfg_target_feature,
         cfg_target_has_atomic,
         cfg_target_thread_local,
@@ -1199,6 +1200,7 @@ symbols! {
         sync,
         sync_trait,
         t32,
+        target_abi,
         target_arch,
         target_endian,
         target_env,