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/philv.ssa

@@ -0,0 +1,34 @@
+# regression test for 1f4ff634
+
+# warning! headaches can occur
+# when trying to figure out what
+# the test is doing!
+
+export
+function w $t0() {
+@start
+@loop
+	%x0 =w phi @start 256, @loop %y0
+	%y0 =w phi @start 128, @loop %y1
+	%y1 =w shr %x0, 1
+	jnz %y1, @loop, @end
+@end
+	ret %x0
+}
+
+export
+function w $t1() {  # swapped phis
+@start
+@loop
+	%y0 =w phi @start 128, @loop %y1
+	%x0 =w phi @start 256, @loop %y0
+	%y1 =w shr %x0, 1
+	jnz %y1, @loop, @end
+@end
+	ret %x0
+}
+
+# >>> driver
+# extern int t0(void), t1(void);
+# int main() { return !(t0() == 1 && t1() == 1);}
+# <<<