Turns out for OSX our data layout was subtly wrong and the LLVM update must have
exposed this. Instead of fixing this I've removed all data layouts from the
compiler to just use the defaults that LLVM provides for all targets. All data
layouts (and a number of dead modules) are removed from the compiler here.
Custom target specifications can still provide a custom data layout, but it is
now an optional key as the default will be used if one isn't specified.
This commit adds support for x86_64-unknown-linux-musl as a target of the
compiler. There's some comments in the commit about some of the more flavorful
flags passed to the linker as it's not quite as trivial as the normal specs.
