Auto merge of #86922 - joshtriplett:target-abi, r=oli-obk

target abi

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.

Update targets to use `target_abi`

All eabi targets have `target_abi = "eabi".`
All eabihf targets have `target_abi = "eabihf"`.
`armv6_unknown_freebsd` and `armv7_unknown_freebsd` have `target_abi = "eabihf"`.
All abi64 targets have `target_abi = "abi64"`.
All ilp32 targets have `target_abi = "ilp32"`.
All softfloat targets have `target_abi = "softfloat"`.
All *-uwp-windows-* targets have `target_abi = "uwp"`.
All spe targets have `target_abi = "spe"`.
All macabi targets have `target_abi = "macabi"`.
aarch64-apple-ios-sim has `target_abi = "sim"`.
`x86_64-fortanix-unknown-sgx` has `target_abi = "fortanix"`.
`x86_64-unknown-linux-gnux32` has `target_abi = "x32"`.

Add FIXME entries for targets for which existing values need to change
once `cfg_target_abi` becomes stable. (All of them are tier 3 targets.)

Add a test for `target_abi` in `--print cfg`.

compiler/rustc_span/src/symbol.rs

@@ -354,6 +354,7 @@ symbols! {
         cfg_eval,
         cfg_panic,
         cfg_sanitize,
+        cfg_target_abi,
         cfg_target_feature,
         cfg_target_has_atomic,
         cfg_target_thread_local,
@@ -1203,6 +1204,7 @@ symbols! {
         sync,
         sync_trait,
         t32,
+        target_abi,
         target_arch,
         target_endian,
         target_env,