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/doc/rv64.txt

@@ -0,0 +1,20 @@
+=========
+RISC-V 64
+=========
+
+- Known issues
+--------------
+
+ABI with structs containing floats is not yet supported.
+
+- Possible improvements
+-----------------------
+
+rv64_isel() could turn compare used only with jnz into b{lt,ge}[u].
+
+- Helpful links
+---------------
+
+RISC-V spec: https://github.com/riscv/riscv-isa-manual/releases/latest/download/riscv-spec.pdf
+ASM manual: https://github.com/riscv-non-isa/riscv-asm-manual/blob/master/riscv-asm.md
+ABI: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc