cmd/link: process is_stmt data into dwarf line tables

To improve debugging, instructions should be annotated with
DWARF is_stmt.  The DWARF default before was is_stmt=1, and
to remove "jumpy" stepping the optimizer was tagging
instructions with a no-position position, which interferes
with the accuracy of profiling information.  This allows
that to be corrected, and also allows more "jumpy" positions
to be annotated with is_stmt=0 (these changes were not made
for 1.10 because of worries about further messing up
profiling).

The is_stmt values are placed in a pc-encoded table and
passed through a symbol derived from the name of the
function and processed in the linker alongside its
processing of each function's pc/line tables.

The only change in binary size is in the .debug_line tables
measured with "objdump -h --section=.debug_line go1.test"
For go1.test, these are 2614 bytes larger,
or 0.72% of the size of .debug_line,
or 0.025% of the file size.

This will increase in proportion to how much the is_stmt
flag is used (toggled).

Change-Id: Ic1f1aeccff44591ad0494d29e1a0202a3c506a7a
Reviewed-on: https://go-review.googlesource.com/93664
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/internal/obj/link.go

@@ -395,6 +395,7 @@ type FuncInfo struct {
 	dwarfLocSym    *LSym
 	dwarfRangesSym *LSym
 	dwarfAbsFnSym  *LSym
+	dwarfIsStmtSym *LSym
 
 	GCArgs   LSym
 	GCLocals LSym