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 possible.
* Testing that we can select an existing triple.
* Testing that triple is not selected if it doesn't exist.
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