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/amd64/targ.c

@@ -0,0 +1,46 @@
+#include "all.h"
+
+Amd64Op amd64_op[NOp] = {
+#define O(op, t, x) [O##op] =
+#define X(nm, zf, lf) { nm, zf, lf, },
+	#include "../ops.h"
+};
+
+static int
+amd64_memargs(int op)
+{
+	return amd64_op[op].nmem;
+}
+
+#define AMD64_COMMON \
+	.gpr0 = RAX, \
+	.ngpr = NGPR, \
+	.fpr0 = XMM0, \
+	.nfpr = NFPR, \
+	.rglob = BIT(RBP) | BIT(RSP), \
+	.nrglob = 2, \
+	.rsave = amd64_sysv_rsave, \
+	.nrsave = {NGPS, NFPS}, \
+	.retregs = amd64_sysv_retregs, \
+	.argregs = amd64_sysv_argregs, \
+	.memargs = amd64_memargs, \
+	.abi0 = elimsb, \
+	.abi1 = amd64_sysv_abi, \
+	.isel = amd64_isel, \
+	.emitfn = amd64_emitfn, \
+
+Target T_amd64_sysv = {
+	.name = "amd64_sysv",
+	.emitfin = elf_emitfin,
+	.asloc = ".L",
+	AMD64_COMMON
+};
+
+Target T_amd64_apple = {
+	.name = "amd64_apple",
+	.apple = 1,
+	.emitfin = macho_emitfin,
+	.asloc = "L",
+	.assym = "_",
+	AMD64_COMMON
+};