Feature request: support +r constraint in extended asm

[didickman]

It would be really great to support the +r constraint in extended asm. For example to support this code: https://github.com/openbsd/src/blob/b3424f948f9275685df5315a2922aad768594479/sys/arch/amd64/include/endian.h#L44

If support is not possible then this type of code could be rewritten.