runtime: remove holes in FUNCDATA tables

Shrinks the text segment size by about 1.5% for the "go", "gofmt",
and "camlistored" commands on linux/amd64.

Before:
$ size go gofmt camlistored
   text	   data	    bss	    dec	    hex	filename
6506842	 136996	 105784	6749622	 66fdb6	go
2376046	  85232	  90984	2552262	 26f1c6	gofmt
17051050	 190256	 130320	17371626	10911ea	camlistored

After:
$ size go gofmt camlistored
   text	   data	    bss	    dec	    hex	filename
6403034	 136996	 105784	6645814	 656836	go
2331118	  85232	  90984	2507334	 264246	gofmt
16842586	 190256	 130320	17163162	105e39a	camlistored

Fixes #8604.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/137790043

src/pkg/runtime/funcdata.h

@@ -10,9 +10,9 @@
 #define PCDATA_ArgSize 0 /* argument size at CALL instruction */
 #define PCDATA_StackMapIndex 1
 
-#define FUNCDATA_ArgsPointerMaps 2 /* garbage collector blocks */
-#define FUNCDATA_LocalsPointerMaps 3
-#define FUNCDATA_DeadValueMaps 4
+#define FUNCDATA_ArgsPointerMaps 0 /* garbage collector blocks */
+#define FUNCDATA_LocalsPointerMaps 1
+#define FUNCDATA_DeadValueMaps 2
 
 // To be used in assembly.
 #define ARGSIZE(n) PCDATA $PCDATA_ArgSize, $n
@@ -27,9 +27,9 @@
 enum {
 	PCDATA_ArgSize = 0,
 	PCDATA_StackMapIndex = 1,
-	FUNCDATA_ArgsPointerMaps = 2,
-	FUNCDATA_LocalsPointerMaps = 3,
-	FUNCDATA_DeadValueMaps = 4,
+	FUNCDATA_ArgsPointerMaps = 0,
+	FUNCDATA_LocalsPointerMaps = 1,
+	FUNCDATA_DeadValueMaps = 2,
 	ArgsSizeUnknown = 0x80000000,
 };
 */