cmd/compile: plumb prologueEnd into DWARF

This marks the first instruction after the prologue for
consumption by debuggers, specifically Delve, who asked
for it.  gdb appears to ignore it, lldb appears to use it.

The bits for end-of-prologue and beginning-of-epilogue
are added to Pos (reducing maximum line number by 4x, to
1048575).  They're added in cmd/internal/obj/<ARCH>.go
(currently x86 only), so the compiler-proper need not
deal with them.

The linker currently does nothing with beginning-of-epilogue,
but the plumbing exists to make it easier in the future.

This also upgrades the line number table to DWARF version 3.

This CL includes a regression in the coverage for
testdata/i22558.gdb-dbg.nexts, this appears to be a gdb
artifact but the fix would be in the preceding CL in the
stack.

Change-Id: I3bda5f46a0ed232d137ad48f65a14835c742c506
Reviewed-on: https://go-review.googlesource.com/110416


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>