feat(asm): assemble static symbols and the whole go-flac AVX2 kernel byte-identically

Assisted-by: Qwen 3.8 Max Preview

asm/operand.go

@@ -41,3 +41,15 @@ func Idx(base, index Reg, scale int, disp int64, size int) Mem {
 func Rip(disp int64, size int) Mem {
 	return Mem{Disp: disp, Size: size}
 }
+
+// sbMem is a memory operand that references a static (SB) symbol.  It encodes
+// as a RIP-relative reference with a placeholder displacement; the encoder
+// records a patch site so the file-level layout can fill in the true rel32
+// once the symbol's address is known.
+type sbMem struct {
+	size   int
+	name   string // static symbol name (the GLOBL identifier)
+	addend int64  // byte offset within the symbol
+}
+
+func (sbMem) isOperand() {}