[mingw/msys only]
Undo long-standing workaround for buggy GNU ld's on mingw/msys; i.e.,
the linker wasn't correctly generating relocatable object files when
the number of relocs exceeded 2^16. Worked around the issue by
hackily splitting up the GHCi object file for the larger packages
('base', 'ObjectIO' and 'win32') into a handful of object files,
each with a manageable number of relocs. Tiresome and error-prone
(but the hack has served us well!)

This commit imposes a restriction on the 'ld' you use to compile
up GHC with; it now has to be ld-2.15.x or later (something GHC
binary dists have shipped with since 6.2.2)