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/_fix4.ssa

@@ -0,0 +1,27 @@
+function $test() {
+@start
+	%x =w copy 3
+	%n =w copy 2
+@loop
+	%c =w ceqw %n, 10000
+	jnz %c, @end, @next
+@next
+	%t =w copy 3
+	%x =w add %x, 2
+@tloop
+	%s =w mul %t, %t
+	%c =w csgtw %s, %x
+	jnz %c, @prime, @test
+@test
+	%r =w rem %x, %t
+	jnz %r, @tnext, @loop
+@tnext
+	%t =w add %t, 2
+	jmp @tloop
+@prime
+	%n =w add %n, 1
+	jmp @loop
+@end
+	storew %x, $a
+	ret
+}