inline asm: Accept 'R' constraint
inline asm: Accept 'R' constraint

This is like 'r' but only accepts the eight legacy regs.  Currently
that's the same as 'r' because we don't support the high ones.

andreiw/tinycc,mingodad/tinycc,code4game/tinycc,andreiw/tinycc,run4flat/tinycc,mingodad/tinycc,avih/tinycc,run4flat/tinycc,run4flat/tinycc,code4game/tinycc,avih/tinycc,avih/tinycc,mirror/tinycc,mingodad/tinycc,mingodad/tinycc,run4flat/tinycc,mirror/tinycc,code4game/tinycc,run4flat/tinycc,avih/tinycc,mirror/tinycc,andreiw/tinycc,mirror/tinycc