When the project instals GDB helper scripts, copy them into
meson-private directory with the right tree layout and write a .gdbinit
script to load them automatically.
Currently it tries to run "cmd" by default in a MSYS2 bash.
Passing "bash" doesn't work since that defaults to WSL and
one has to pass an absolute path to bash instead, and even then
one misses out on the PS1 override.
In case $SHELL is set and the contained path exists prefer it even if
we are on Windows.
To make the PS1 override work we can't use Unix paths in Python since
we might be on Windows, so move the .bashrc check into the temporary
bash script itself.
This makes "meson devenv" work the same under MSYS2 as on Linux.
"meson setup" is resolving symlinks for the build directory in
validate_core_dirs. For consistency with it, do the same when
the build directory is passed via -C to devenv, dist, init, install
and test.
This ensures for example that the path to test dependencies is
computed correctly in "meson test".
Fixes: #8765
Xavier Claessens