cmd: initial compiler+linker support for DWARF5 .debug_addr

This patch rolls the main .debug_info DWARF section from version 4 to
version 5, and also introduces machinery in the Go compiler and linker
for taking advantage of the DWARF5 ".debug_addr" section for
subprogram DIE "high" and "low" PC attributes. All functionality is
gated by GOEXPERIMENT=dwarf5.

For the compiler portion of this patch, we add a new DIE attribute
form "DW_FORM_addrx", which accepts as an argument a function (text)
symbol.  The dwarf "putattr" function is enhanced to handle this
format by invoking a new dwarf context method "AddIndirectTextRef".
Under the hood, this method invokes the Lsym method WriteDwTxtAddrx,
which emits a new objabi.R_DWTXTADDR_* relocation. The size of the
relocation is dependent on the number of functions in the package; we
pick a size that is just big enough for the largest func index.

In the linker portion of this patch, we now switch over to writing out
a version number of 5 (instead of 4) in the compile unit header (this
is required if we want to use addrx attributes). In the parallel portion
of DWARF gen, within each compilation unit we scan subprogram DIEs to
look for R_DWTXTADDR_* relocations, and when we find such a reloc,
we assign a slot in the .debug_addr section for the func targeted.
After the parallel portion is complete, we then walk through all of the
compilation units to assign a value to their DW_AT_addr_base attribute,
which points to the portion of the single .debug_addr section containing
the text addrs for that compilation unit.

Note that once this patch is in, programs built with GOEXPERIMENT=dwarf5
will have broken/damaged DWARF info; in particular, since we've changed
only the CU and subprogram DIEs and haven't incorported the other
changes mandated by DWARF5 (ex: .debug_ranges => .debug_rnglists)
a lot of the variable location info will be missing/incorrect. This
will obviously change in subsequent patches.

Note also that R_DWTXTADDR_* can't be used effectively for lexical
scope DIE hi/lo PC attrs, since there isn't a viable way to encode
"addrx + constant" in the attribute value (you would need a new entry
for each attr endpoint in .debug_addr, which would defeat the point).

Updates #26379.

Change-Id: I2dfc45c9a8333e7b2a58f8e3b88fc8701fefd006
Reviewed-on: https://go-review.googlesource.com/c/go/+/635337
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/internal/obj/link.go

@@ -1158,6 +1158,7 @@ type Link struct {
 	PosTable           src.PosTable
 	InlTree            InlTree // global inlining tree used by gc/inl.go
 	DwFixups           *DwarfFixupTable
+	DwTextCount        int
 	Imports            []goobj.ImportedPkg
 	DiagFunc           func(string, ...interface{})
 	DiagFlush          func()