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````
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