[flang] Add --gcc-toolchain and --gcc-install-dir options to flang. (#87360)

The `--gcc-toolchain` and `--gcc-install-dir` option were previously only visible to the Clang driver, but not Flang. The

[flang] Add --gcc-toolchain and --gcc-install-dir options to flang. (#87360)

The `--gcc-toolchain` and `--gcc-install-dir` option were previously only visible to the Clang driver, but not Flang. These determine which assembler, linker, and libraries to use, e.g. for cross-compiling, and therefore are relevant for Flang as well.

Tests are implemented using a mock GCC installation in `basic_cross_linux_tree` copied over from Clang's tests. The Clang driver already contains tests with `--driver-mode=flang` but `flang-new` is an entirely different executable (containing the `-fc1` stage) that should be tested as well. While not all files in `basic_cross_linux_tree` are strictly needed for testing those two driver flags, they will be necessarily needed for future added flags such as `--rtlib`.
 
Also remove the entry `*.o` in flang's `.gitignore` since `crt*.o` files are needed in the GCC mock installation.

Fixes #86729

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