builtin: use == 0 instead of isnil()

Do not rely on C inlining that may not happen for a *very* frequently
called fn in lower level code.

vlib/builtin/builtin_nix.c.v

@@ -110,7 +110,7 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 				cmd := 'addr2line -e $executable $addr'
 				// taken from os, to avoid depending on the os module inside builtin.v
 				f := C.popen(&char(cmd.str), c'r')
-				if isnil(f) {
+				if f == 0 {
 					eprintln(sframe)
 					continue
 				}