6l: move mapped symbol table lower in memory

Allows binary to run on some Linux system.

Fix for issue 365.

R=rsc
CC=golang-dev
https://golang.org/cl/199096

src/cmd/5l/asm.c

@@ -68,7 +68,7 @@ asmb(void)
 	int np;
 	vlong va, fo, w, symo;
 	int strtabsize;
-	vlong symdatva = 0x99LL<<24;
+	vlong symdatva = SYMDATVA;
 	Optab *o;
 
 	strtabsize = 0;

src/cmd/6l/asm.c

@@ -439,7 +439,7 @@ asmb(void)
 	int a, dynsym;
 	uchar *op1;
 	vlong vl, va, startva, fo, w, symo, elfsymo, elfstro, elfsymsize, machlink;
-	vlong symdatva = 0x99LL<<32;
+	vlong symdatva = SYMDATVA;
 	ElfEhdr *eh;
 	ElfPhdr *ph, *pph;
 	ElfShdr *sh;

src/cmd/ld/lib.h

@@ -28,6 +28,9 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+// This magic number also defined in src/pkg/runtime/symtab.c in SYMCOUNTS
+#define SYMDATVA 0x99LL<<24
+
 typedef struct Library Library;
 struct Library
 {

src/pkg/runtime/symtab.c

@@ -18,13 +18,8 @@
 
 // TODO(rsc): Move this *under* the text segment.
 // Then define names for these addresses instead of hard-coding magic ones.
-#ifdef _64BIT
-#define SYMCOUNTS ((int32*)(0x99LL<<32))	// known to 6l
-#define SYMDATA ((byte*)(0x99LL<<32) + 8)
-#else
-#define SYMCOUNTS ((int32*)(0x99LL<<24))	// known to 8l
+#define SYMCOUNTS ((int32*)(0x99LL<<24))   // known to 6l, 8l; see src/cmd/ld/lib.h
 #define SYMDATA ((byte*)(0x99LL<<24) + 8)
-#endif
 
 typedef struct Sym Sym;
 struct Sym