fix(qbe): any-child terminator check in QBE_IsTerminator for hBlock — QBE SSA now fully valid

- QBE_IsTerminator(hBlock) checks ALL children, not just last
- Removed legacy c_backend.bux and nim_backend.bux from _selfhost/src/
- QBE self-hosting QBE phase: PASSES (SSA valid, assembled to .s)
- Remaining: linker errors from missing String_Eq/StringBuilder in runtime

vendor/qbe/test/_spill3.ssa

@@ -0,0 +1,24 @@
+# make sure comparisons
+# never get their two
+# operands in memory
+# run with NReg == 3, or
+# adapt it!
+
+function $test() {
+@start
+	%a =w loadw $a
+	%b =w loadw $a
+
+@loop
+	%c =w phi @start 0, @loop %f
+	%d =w phi @start 0, @loop %g
+	%e =w phi @start 0, @loop %h
+	%f =w add %c, %d
+	%g =w add %c, %e
+	%h =w add %e, %d
+	%x =w cslew %a, %b
+	jnz %x, @loop, @end
+
+@end
+	ret
+}