[Bug d/89255] libphobos.unittests multilib handling broken

[ibuclaw at gdcproject dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89255

--- Comment #1 from Iain Buclaw <ibuclaw at gdcproject dot org> ---
I'll use this PR as a reference to adding a compiler switch for building
modules in a "unittest runner" mode.  This to allow making it possible to build
all modules using dg-runtest without getting linker errors (as the modules
themselves exist in libphobos).

Currently, all modules are compiled with all unittests bundled into a
libgphobos_t.{so,a} test library before dejagnu kicks in.  The unittest runner
is just a thin wrapper around executing `./unittest` and then executing it
again in a loop, passing as an argument every module printed to stdout.