[flang][driver] Bring --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
```

* Test: `LIT_FILTER="Flang :: Driver/gcc-triple.f90" ninja check-flang`
  * Copied `flang/test/Driver/Inputs/fedora_39_tree` from `clang/test/Driver/Inputs/fedora_39_tree`.
  * Testing what default triple is selected when two are given.
  * Testing that we can select an existing triple.
  * Testing that triple is not selected if it doesn't exist.

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>,

flang/test/Driver/Inputs/fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13/crtbegin.o

@@ -0,0 +1,18 @@
+!! UNSUPPORTED: system-windows
+
+!! Test that --gcc-triple option is working as expected.
+
+! RUN: %flang -v --sysroot=%S/Inputs/fedora_39_tree 2>&1 | FileCheck %s --dump-input=always --check-prefix=DEFAULT_TRIPLE
+! DEFAULT_TRIPLE: {{^}}Found candidate GCC installation:
+! DEFAULT_TRIPLE: fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13
+! DEFAULT_TRIPLE: {{^}}Found candidate GCC installation:
+! DEFAULT_TRIPLE: fedora_39_tree/usr/lib/gcc/x86_64-redhat-linux/13
+! DEFAULT_TRIPLE: {{^}}Selected GCC installation:
+! DEFAULT_TRIPLE: fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13
+
+! RUN: %flang -v --sysroot=%S/Inputs/fedora_39_tree --gcc-triple=x86_64-redhat-linux 2>&1 | FileCheck %s --check-prefix=TRIPLE_EXISTS
+! TRIPLE_EXISTS: {{^}}Selected GCC installation:
+! TRIPLE_EXISTS: fedora_39_tree/usr/lib/gcc/x86_64-redhat-linux/13
+
+! RUN: %flang -v --sysroot=%S/Inputs/fedora_39_tree --gcc-triple=x86_64-foo-linux 2>&1 | FileCheck %s --check-prefix=TRIPLE_DOES_NOT_EXISTS
+! TRIPLE_DOES_NOT_EXISTS-NOT: x86_64-foo-linux