Add support for const operands and options to global_asm!
On x86, the default syntax is also switched to Intel to match asm!.
Currently `global_asm!` only supports `const` operands and the `att_syntax` option. In the future, `sym` operands will also be supported. However there is no plan to support any of the other operand types or options since they don't make sense in the context of `global_asm!`.
r? `@nagisa`