Config file: Use a separate section for multilib targets

[kinke]

No description provided.

[kinke]

This transitions from a single merged default section:

default:
{
    switches = [
        "-defaultlib=phobos2-ldc,druntime-ldc",
        "-L--no-warn-search-mismatch",
    ];
    post-switches = [
        "-I%%ldcbinarypath%%/../import",
    ];
    lib-dirs = [
        "%%ldcbinarypath%%/../lib",
        "%%ldcbinarypath%%/../lib32",
    ];
    rpath = "%%ldcbinarypath%%/../lib:%%ldcbinarypath%%/../lib32";
};

to 2 distinct sections, the multilib one overriding lib dir and rpath:

default:
{
    switches = [
        "-defaultlib=phobos2-ldc,druntime-ldc",
    ];
    post-switches = [
        "-I%%ldcbinarypath%%/../import",
    ];
    lib-dirs = [
        "%%ldcbinarypath%%/../lib",
    ];
    rpath = "%%ldcbinarypath%%/../lib";
};

"i686-.*-linux-gnu":
{
    lib-dirs = [
        "%%ldcbinarypath%%/../lib32",
    ];
    rpath = "%%ldcbinarypath%%/../lib32";
};

This resolves issues wrt. the --no-warn-search-mismatch linker flag (LLD: no support, gold: #3262 (comment)).

[kinke]

Merging eagerly as prerequisite for #3262.