Support volatile

There are couple of options for supporting volatile because it can't be exactly like GCC's two separate keywords `asm volatile ("template")`.

 1. Create a second macro like `gcc_asm_volatile!("template")`
 2. Allow `volatile` keyword before template `gcc_asm!(volatile "template")`
 3. Do it like rust `gcc_asm!("template" :::: "volatile")`

I think I prefer 2.