Implement C/C++ export for registers with field index gaps

Reserved or unassigned bits in a register must be exported explicitly in the C and C++ idiomatic representations.

eg.

# r1[0:0] implicitly unassigned 
f1 = r1[1:3]
# r1[4:4] implicitly unassigned
f2 = r1[5:5]
# r1[6:7] implicitly unassigned