give the abi-relevant target feature tests better names

Author: RalfJung

tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.rs renamed to tests/ui/target-feature/abi-incompatible-target-feature-attribute.rs

@@ -1,5 +1,5 @@
 error: target feature `d` cannot be enabled with `#[target_feature]`: this feature is incompatible with the target ABI
-  --> $DIR/forbidden-hardfloat-target-feature-attribute.rs:10:18
+  --> $DIR/abi-incompatible-target-feature-attribute.rs:10:18
    |
 LL | #[target_feature(enable = "d")]
    |                  ^^^^^^^^^^^^

tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.stderr renamed to tests/ui/target-feature/abi-incompatible-target-feature-attribute.stderr

@@ -1,3 +1,6 @@
+//! `x87` is a required target feature on some x86 targets, but not on this one as this one
+//! uses soft-floats. So ensure disabling the target feature here (which is a NOP) does
+//! not trigger a warning.
 //@ compile-flags: --target=x86_64-unknown-none --crate-type=lib
 //@ needs-llvm-components: x86
 //@ compile-flags: -Ctarget-feature=-x87

tests/ui/target-feature/forbidden-hardfloat-target-feature-flag.rs renamed to tests/ui/target-feature/abi-incompatible-target-feature-flag-enable.rs

@@ -1,3 +1,5 @@
+//! Enabling a target feature that is anyway required changes nothing, so this is allowed
+//! for `#[target_feature]`.
 //@ compile-flags: --target=x86_64-unknown-none --crate-type=lib
 //@ needs-llvm-components: x86
 //@ build-pass