Use define from if_var.h to access a field inside struct if_data,
that resides in struct ifnet.

Sponsored by:	Nginx, Inc.
The "SUB" instruction used in getcc() actually does 'x -= y' so use the
proper constraint for 'x'. The "+r" constraint indicates that 'x' is an
input and output register operand.

While here generate code for different variants of getcc() using a macro
GETCC(sz) where 'sz' indicates the operand size.

Update the status bits in %rflags when emulating AND and OR opcodes.

Reviewed by:	grehan
Remove FDT option, since it is in every file, which includes this one.