operand: include '+0' in named symbol references

Intended to address an asmdecl error.

  [amd64] Butterfly: use of unnamed argument 0(FP); offset 0 is x0+0(FP)

Updates #24

examples/data/data.s

@@ -2,7 +2,7 @@
 
 #include "textflag.h"
 
-DATA bytes<>(SB)/8, $0x0011223344556677
+DATA bytes<>+0(SB)/8, $0x0011223344556677
 DATA bytes<>+8(SB)/8, $"strconst"
 DATA bytes<>+16(SB)/4, $(3.1415927)
 DATA bytes<>+24(SB)/8, $(3.141592653589793)
@@ -14,8 +14,8 @@ GLOBL bytes<>(SB), RODATA|NOPTR, $40
 
 // func DataAt(i int) byte
 TEXT ·DataAt(SB), NOSPLIT, $0-9
-	MOVQ i(FP), AX
-	LEAQ bytes<>(SB), CX
+	MOVQ i+0(FP), AX
+	LEAQ bytes<>+0(SB), CX
 	MOVB (CX)(AX*1), AL
 	MOVB AL, ret+8(FP)
 	RET