rustc 1.27.0-nightly breaks the debug version

[Xudong-Huang]

rustc 1.27.0-nightly generate unwanted code for naked functions, this affect the logic of swap_registers by using inline assembly.

// why save the rcx and rdx in stack? this will overwite something!
// the naked function should only use the asm block, debug version breaks
// since rustc 1.27.0-nightly, we have to use O2 level optimization
        :
        : "{rcx}"(out_regs), "{rdx}"(in_regs) 
        : "memory"
        : "volatile");

[Xudong-Huang]

rustc 1.27.0-nightly (79252ff4e 2018-04-29) works fine
rustc 1.27.0-nightly (f9bfe840f 2018-05-05) not work!