Since CL 115060044, mkanames declares an empty
array in anames8.c and anames6.c, which is not
valid for the Plan 9 compiler.

char* cnames8[] = {
};

This change makes mkanames not declaring the
cnames array when no C_ constants are found.

LGTM=iant
R=minux, iant
CC=golang-codereviews
https://golang.org/cl/117680043