x86 C ABI struct return does not work on windows/mac for structs < 8 bytes

[brson]

C returns the structs packed into %eax but our code appears to load the fields from %al and %dl for byte fields.

See run-pass/extern-return-TwoU8s and run-pass/extern-return-TwoU16s.

[brson]

Mac appears to follow the same ABI as windows here.

[pnkfelix]

visiting for bug traige, 2013-08-26.

Nominating for maturity milestone 3: Feature Complete.

[pnkfelix]

accepted for feature complete

[nikomatsakis]

Those test cases work now (I fixed this as part of my extern fn work)