This adds a flag -split-sections that does similar things to -split-objs, but
using sections in single object files instead of relying on the Satanic
Splitter and other abominations. This is very similar to the GCC flags
-ffunction-sections and -fdata-sections.
The --gc-sections linker flag, which allows unused sections to actually
be removed, is added to all link commands (if the linker supports it) so that
space savings from having base compiled with sections can be realized.
Supported both in LLVM and the native code-gen, in theory for all
architectures, but really tested on x86 only.
In the GHC build, a new SplitSections variable enables -split-sections for
relevant parts of the build.