Added -S flag to print go source file line above corresponding disassembly:

$ go tool objdump -S -s main.main fmthello
TEXT main.main(SB) /home/rugginoso/Documents/src/go/src/cmd/objdump/testdata/fmthello.go
func main() {
  0x47d450		64488b0c25f8ffffff	FS MOVQ FS:0xfffffff8, CX
  0x47d459		483b6110		CMPQ 0x10(CX), SP
  0x47d45d		7631			JBE 0x47d490
  0x47d45f		4883ec18		SUBQ $0x18, SP
  0x47d463		48896c2410		MOVQ BP, 0x10(SP)
  0x47d468		488d6c2410		LEAQ 0x10(SP), BP
	Println("hello, world")
  0x47d46d		488d0563b00200		LEAQ 0x2b063(IP), AX
  0x47d474		48890424		MOVQ AX, 0(SP)
  0x47d478		48c74424080c000000	MOVQ $0xc, 0x8(SP)
  0x47d481		e81a000000		CALL main.Println(SB)
}
  0x47d486		488b6c2410		MOVQ 0x10(SP), BP
  0x47d48b		4883c418		ADDQ $0x18, SP
  0x47d48f		c3			RET
func main() {
  0x47d490		e8ebf1fcff		CALL runtime.morestack_noctxt(SB)
  0x47d495		ebb9			JMP main.main(SB)

Execution time:

$ time go tool objdump testdata/fmthello > /dev/null
real	0m0.430s
user	0m0.440s
sys	0m0.000s

$ time go tool objdump -S testdata/fmthello > /dev/null
real	0m0.471s
user	0m0.476s
sys	0m0.012s

Fixes #18245

Change-Id: I9b2f8338f9ee443c1352efd270d3ba85e3dd9b78
Reviewed-on: https://go-review.googlesource.com/37953
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>