rustc_target: RISC-V "Zfinx" is incompatible with {ILP32,LP64}[FD] ABIs

Because RISC-V Calling Conventions note that:

> This means code targeting the Zfinx extension always uses the ILP32,
> ILP32E or LP64 integer calling-convention only ABIs as there is no
> dedicated hardware floating-point register file.

{ILP32,LP64}[FD] ABIs with hardware floating-point calling conventions
are incompatible with the "Zfinx" extension.

This commit adds "zfinx" to the incompatible feature list to those ABIs
and tests whether trying to add "zdinx" (that is analogous to "zfinx" but
in double-precision) on a LP64D ABI configuration results in an error
(it also tests extension implication; "Zdinx" requires "Zfinx" extension).

Link: RISC-V psABI specification version 1.0
<https://github.com/riscv-non-isa/riscv-elf-psabi-doc/releases/tag/v1.0>

Author: a4lg

compiler/rustc_target/src/target_features.rs

@@ -894,12 +894,12 @@ impl Target {
                 // about what the intended ABI is.
                 match &*self.llvm_abiname {
                     "ilp32d" | "lp64d" => {
-                        // Requires d (which implies f), incompatible with e.
-                        FeatureConstraints { required: &["d"], incompatible: &["e"] }
+                        // Requires d (which implies f), incompatible with e and zfinx.
+                        FeatureConstraints { required: &["d"], incompatible: &["e", "zfinx"] }
                     }
                     "ilp32f" | "lp64f" => {
-                        // Requires f, incompatible with e.
-                        FeatureConstraints { required: &["f"], incompatible: &["e"] }
+                        // Requires f, incompatible with e and zfinx.
+                        FeatureConstraints { required: &["f"], incompatible: &["e", "zfinx"] }
                     }
                     "ilp32" | "lp64" => {
                         // Requires nothing, incompatible with e.

tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute-2.rs

@@ -0,0 +1,12 @@
+//! Ensure ABI-incompatible features cannot be enabled via `#[target_feature]`.
+//@ compile-flags: --target=riscv64gc-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: riscv
+#![feature(no_core, lang_items, riscv_target_feature)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+#[target_feature(enable = "zdinx")]
+//~^ERROR: cannot be enabled with
+pub unsafe fn my_fun() {}

tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute-2.stderr

@@ -0,0 +1,8 @@
+error: target feature `zfinx` cannot be enabled with `#[target_feature]`: this feature is incompatible with the target ABI
+  --> $DIR/forbidden-hardfloat-target-feature-attribute-2.rs:10:18
+   |
+LL | #[target_feature(enable = "zdinx")]
+   |                  ^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+