The phoney gcc that we construct for multilib was missing the
`$out/include/c++` directory which `cc-wrapper` needs to pass as an
`-isystem` to `clang`.
Closes#221891
The phoney gcc that we construct for multilib was missing the
`langCC` attribute, which `cc-wrapper` needs in order to decide
whether or not to add gcc's `libstdc++` headers as an `-isystem` for
`clang`.
Our gcc_multi and glibc_multi expressions merge together a
32-bit-targeted and 64-bit-targeted gcc. However they do not thread
through the passthru.libgcc from these merged gccs.
This commit corrects that.
It also extends passthru.libgcc to allow a *list* rather than just a
single outpath.
Resolves part of #221891 (at least getting it back to the error
message it gave before).
