rustc_target: RISC-V: Add BF16 extensions

This commit adds three ratified unprivileged RISC-V extensions related to
BFloat16 (BF16) handling.

Although that they are far from stabilization due to ABI issues, they are
optional extensions of the RVA23U64 profile (application-class processor
profile) and going to be discoverable from the Linux kernel
(as of version 6.15-rc4).

This commit mainly prepares runtime detection of those extensions.

Author: a4lg

compiler/rustc_target/src/target_features.rs

@@ -526,6 +526,7 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zcmop", Unstable(sym::riscv_target_feature), &["zca"]),
     ("zdinx", Unstable(sym::riscv_target_feature), &["zfinx"]),
     ("zfa", Unstable(sym::riscv_target_feature), &["f"]),
+    ("zfbfmin", Unstable(sym::riscv_target_feature), &["f"]), // and a subset of Zfhmin
     ("zfh", Unstable(sym::riscv_target_feature), &["zfhmin"]),
     ("zfhmin", Unstable(sym::riscv_target_feature), &["f"]),
     ("zfinx", Unstable(sym::riscv_target_feature), &["zicsr"]),
@@ -565,6 +566,8 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zve64d", Unstable(sym::riscv_target_feature), &["zve64f", "d"]),
     ("zve64f", Unstable(sym::riscv_target_feature), &["zve32f", "zve64x"]),
     ("zve64x", Unstable(sym::riscv_target_feature), &["zve32x", "zvl64b"]),
+    ("zvfbfmin", Unstable(sym::riscv_target_feature), &["zve32f"]),
+    ("zvfbfwma", Unstable(sym::riscv_target_feature), &["zfbfmin", "zvfbfmin"]),
     ("zvfh", Unstable(sym::riscv_target_feature), &["zvfhmin", "zve32f", "zfhmin"]), // Zvfh ⊃ Zvfhmin
     ("zvfhmin", Unstable(sym::riscv_target_feature), &["zve32f"]),
     ("zvkb", Unstable(sym::riscv_target_feature), &["zve32x"]),

tests/ui/check-cfg/target_feature.stderr

@@ -324,6 +324,7 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `zcmop`
 `zdinx`
 `zfa`
+`zfbfmin`
 `zfh`
 `zfhmin`
 `zfinx`
@@ -363,6 +364,8 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `zve64d`
 `zve64f`
 `zve64x`
+`zvfbfmin`
+`zvfbfwma`
 `zvfh`
 `zvfhmin`
 `zvkb`