Rollup merge of #109155 - taiki-e:riscv64-fuchsia-fix-llvm-target, r=tmandry

Fix riscv64 fuchsia LLVM target name

Currently, riscv64gc-unknown-fuchsia (added in #108722) sets riscv64*gc*-unknown-fuchsia as the LLVM target.

1716932743/compiler/rustc_target/src/spec/riscv64gc_unknown_fuchsia.rs (L5)

However, riscv64*gc*-\* is not a valid LLVM target and causes the following error.

```console
$ rustc --print cfg --target riscv64gc-unknown-fuchsia
error: could not create LLVM TargetMachine for triple: riscv64gc-unknown-fuchsia: No available targets are compatible with triple "riscv64gc-unknown-fuchsia"
```

As with other RISC-V targets, the LLVM target should use riscv64-\*, not riscv64*gc*-\*.

1716932743/compiler/rustc_target/src/spec/riscv64gc_unknown_freebsd.rs (L5)
1716932743/compiler/rustc_target/src/spec/riscv64gc_unknown_linux_gnu.rs (L5)

I confirmed that riscv64-unknown-fuchsia is recognized as a valid LLVM target by using custom targets.

```console
# create a custom target with `"llvm-target": "riscv64-unknown-fuchsia" from no-std riscv64gc target.
$ rustc --print target-spec-json -Z unstable-options --target riscv64gc-unknown-none-elf | grep -v is-builtin | sed 's/"llvm-target".*/"llvm-target": "riscv64-unknown-fuchsia",/' > riscv64gc-unknown-fuchsia.json

$ rustc --print cfg --target riscv64gc-unknown-fuchsia.json
debug_assertions
panic="abort"
target_abi=""
target_arch="riscv64"
target_endian="little"
target_env=""
target_feature="a"
target_feature="c"
target_feature="d"
target_feature="f"
target_feature="m"
...

$ cat riscv64gc-unknown-fuchsia.json
{
  "arch": "riscv64",
  "code-model": "medium",
  "cpu": "generic-rv64",
  "data-layout": "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128",
  "eh-frame-header": false,
  "emit-debug-gdb-scripts": false,
  "features": "+m,+a,+f,+d,+c",
  "linker": "rust-lld",
  "linker-flavor": "ld.lld",
  "llvm-abiname": "lp64d",
  "llvm-target": "riscv64-unknown-fuchsia",
  "max-atomic-width": 64,
  "panic-strategy": "abort",
  "relocation-model": "static",
  "supported-sanitizers": [
    "kernel-address"
  ],
  "target-pointer-width": "64"
}

# Check the current master's LLVM target name causes an error
$ sed -i 's/riscv64-unknown-fuchsia/riscv64gc-unknown-fuchsia/' riscv64gc-unknown-fuchsia.json
$ rustc --print cfg --target riscv64gc-unknown-fuchsia.json
error: could not create LLVM TargetMachine for triple: riscv64gc-unknown-fuchsia: No available targets are compatible with triple "riscv64gc-unknown-fuchsia"
```

r? ````@tmandry````

compiler/rustc_target/src/spec/riscv64gc_unknown_fuchsia.rs

@@ -2,7 +2,7 @@
 
 pub fn target() -> Target {
     Target {
-        llvm_target: "riscv64gc-unknown-fuchsia".into(),
+        llvm_target: "riscv64-unknown-fuchsia".into(),
         pointer_width: 64,
         data_layout: "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128".into(),
         arch: "riscv64".into(),