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() {
+}

src/test/ui/feature-gates/feature-gate-cfg-target-abi.rs

@@ -0,0 +1,11 @@
+#[cfg(target_abi = "x")] //~ ERROR `cfg(target_abi)` is experimental
+#[cfg_attr(target_abi = "x", x)] //~ ERROR `cfg(target_abi)` is experimental
+struct Foo(u64, u64);
+
+#[cfg(not(any(all(target_abi = "x"))))] //~ ERROR `cfg(target_abi)` is experimental
+fn foo() {}
+
+fn main() {
+    cfg!(target_abi = "x");
+    //~^ ERROR `cfg(target_abi)` is experimental and subject to change
+}

src/test/ui/feature-gates/feature-gate-cfg-target-abi.stderr

@@ -0,0 +1,39 @@
+error[E0658]: `cfg(target_abi)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-abi.rs:2:12
+   |
+LL | #[cfg_attr(target_abi = "x", x)]
+   |            ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #80970 <https://github.com/rust-lang/rust/issues/80970> for more information
+   = help: add `#![feature(cfg_target_abi)]` to the crate attributes to enable
+
+error[E0658]: `cfg(target_abi)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-abi.rs:1:7
+   |
+LL | #[cfg(target_abi = "x")]
+   |       ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #80970 <https://github.com/rust-lang/rust/issues/80970> for more information
+   = help: add `#![feature(cfg_target_abi)]` to the crate attributes to enable
+
+error[E0658]: `cfg(target_abi)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-abi.rs:5:19
+   |
+LL | #[cfg(not(any(all(target_abi = "x"))))]
+   |                   ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #80970 <https://github.com/rust-lang/rust/issues/80970> for more information
+   = help: add `#![feature(cfg_target_abi)]` to the crate attributes to enable
+
+error[E0658]: `cfg(target_abi)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-abi.rs:9:10
+   |
+LL |     cfg!(target_abi = "x");
+   |          ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #80970 <https://github.com/rust-lang/rust/issues/80970> for more information
+   = help: add `#![feature(cfg_target_abi)]` to the crate attributes to enable
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/tools/tidy/src/pal.rs

@@ -122,6 +122,7 @@ fn check_cfgs(
 
         let contains_platform_specific_cfg = cfg.contains("target_os")
             || cfg.contains("target_env")
+            || cfg.contains("target_abi")
             || cfg.contains("target_vendor")
             || cfg.contains("unix")
             || cfg.contains("windows");