5l, runtime: remove softfloat emulation code when not used

This leads to ~30kB improvement on code size for ARM machines with VFP/NEON.
Example: go test -c math
       GOARM=5  GOARM=6
  Old: 1884200  1839144
  New: 1884165  1805245
  -:        35    33899

R=rsc, bradfitz, dave, kai.backman
CC=golang-dev
https://golang.org/cl/5975060

src/cmd/5l/obj.c

@@ -63,13 +63,6 @@ Header headers[] = {
  *	-Hlinux -Tx -Rx			is linux elf
  */
 
-static char*
-linkername[] =
-{
-	"runtime.softfloat",
-	"math.sqrtGoC",
-};
-
 void
 usage(void)
 {
@@ -80,7 +73,7 @@ usage(void)
 void
 main(int argc, char *argv[])
 {
-	int c, i;
+	int c;
 	char *p, *name, *val;
 
 	Binit(&bso, 1, OWRITE);
@@ -250,9 +243,8 @@ main(int argc, char *argv[])
 	loadlib();
 
 	// mark some functions that are only referenced after linker code editing
-	// TODO(kaib): this doesn't work, the prog can't be found in runtime
-	for(i=0; i<nelem(linkername); i++)
-		mark(lookup(linkername[i], 0));
+	if(debug['F'])
+		mark(rlookup("_sfloat", 0));
 	deadcode();
 	if(textp == nil) {
 		diag("no code");

src/pkg/runtime/asm_arm.s

@@ -58,17 +58,6 @@ TEXT _rt0_arm(SB),7,$-4
 	MOVW	$1234, R0
 	MOVW	$1000, R1
 	MOVW	R0, (R1)	// fail hard
-	B	runtime·_dep_dummy(SB)	// Never reached
-
-// TODO(kaib): remove these once i actually understand how the linker removes symbols
-// pull in dummy dependencies
-TEXT runtime·_dep_dummy(SB),7,$0
-	BL	_div(SB)
-	BL	_divu(SB)
-	BL	_mod(SB)
-	BL	_modu(SB)
-	BL	_modu(SB)
-	BL	_sfloat(SB)
 
 TEXT runtime·breakpoint(SB),7,$0
 	// no breakpoint yet; let program exit