Add a `-C no-gc-sections` option to rustc

[jansegre]

I have an issue when static linking to an external library on OS X 64-bit (in my case luajit) that is known to have problems when -dead_strip is passed to the linker due to some code being written in asm. A brief talk in the IRC lead to @cmr suggesting me to open this issue. The name no-gc-sections is for matching the gnu ld --gc-sections nomenclature.

I'm keeping it brief I can give more details if requested.
Edit: PS: I'm manually linking as temporary fix.

[steveklabnik]

Triage: no change

[pnkfelix]

-C link-dead-code covers this at this point, I think. At least, the way its implemented is that it causes the compiler to stop including --gc-sections in its linker invocation.

[alexcrichton]

Indeed!