Fix targets for Darwin PowerPC

[barracuda156]

See: #300 (comment)

@thepowersgang Could you please check this?

P. S. As I mention in the topic, Darwin on ppc32 may need 4-byte bools, since Apple ABI defines that: http://personal.denison.edu/~bressoud/cs281-s07/MacOSXLowLevelABI.pdf
Not sure if that should apply to Rust code though.

[thepowersgang]

Seems reasonable, although - do you want to wait until the PPC support is working?

[barracuda156]

@thepowersgang I think this can be merged, as long as we are okay with targets (IMO they are correct), since mrustc itself builds and runs, what is broken is upstream rust, which can be fixed separately. Having ppc targets fixed may also be helpful in a case someone else will join the endeavor to fix rust for ppc, FWIW.
But of course this is up to you to decide.