Bing --gcc-triple to flang

When there are multiple gcc versions installed, we want `flang` to be able
to find the right one based on the triple. Here's `flang` selecting an
unwanted `gcc` candidate installation, namely
`/usr/lib/gcc/x86_64-linux-gnu/15`:

```
~/src/llvm-project/main/build-RelWithDebInfo > ./bin/flang -v
flang version 22.0.0custombuild
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /home/fedora/src/llvm-project/main/build-RelWithDebInfo/bin
System configuration file directory: /etc/clang/
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
```

When passing `--gcc-triple=x86_64-redhat-linux` we get the desired gcc
candidate installation:

```
~/src/llvm-project/main/build-RelWithDebInfo > ./bin/flang --gcc-triple=x86_64-redhat-linux -v
flang version 22.0.0custombuild
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /home/fedora/src/llvm-project/main/build-RelWithDebInfo/bin
System configuration file directory: /etc/clang/
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/15
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
```

Author: kwk

clang/include/clang/Driver/Options.td

@@ -741,6 +741,7 @@ def gcc_toolchain : Joined<["--"], "gcc-toolchain=">, Flags<[NoXarchOption]>,
     "Specify a directory where Flang can find 'lib{,32,64}/gcc{,-cross}/$triple/$version'. "
     "Flang will use the GCC installation with the largest version">;
 def gcc_triple_EQ : Joined<["--"], "gcc-triple=">,
+  Visibility<[ClangOption, FlangOption]>,
   HelpText<"Search for the GCC installation with the specified triple.">;
 def CC : Flag<["-"], "CC">, Visibility<[ClangOption, CC1Option]>,
   Group<Preprocessor_Group>,