Don't pass -ffunction-sections or -fdata-sections on Darwin targets

[comex]

On GNU targets, these flags are necessary to allow the linker to strip unused symbols. On Darwin, they aren't necessary for that, and in fact don't seem to have any effect. Normally they're harmless, but if the user passes -fembed-bitcode in CFLAGS, Xcode's version of clang gets upset:

clang: error: -ffunction-sections is not supported with -fembed-bitcode
clang: error: -fdata-sections is not supported with -fembed-bitcode

Thus, it would be best to simply omit these flags on Darwin.

[alexcrichton]

Sounds reasonable to me!