cmd: fix DWARF gen bug with packages that use assembly

When the compiler builds a Go package with DWARF 5 generation enabled,
it emits relocations into various generated DWARF symbols (ex:
SDWARFFCN) that use the R_DWTXTADDR_* flavor of relocations. The
specific size of this relocation is selected based on the total number
of functions in the package -- if the package is tiny (just a couple
funcs) we can use R_DWTXTADDR_U1 relocs (which target just a byte); if
the package is larger we might need to use the 2-byte or 3-byte flavor
of this reloc.

Prior to this patch, the strategy used to pick the right relocation
size was flawed in that it didn't take into account packages with
assembly code. For example, if you have a package P with 200 funcs
written in Go source and 200 funcs written in assembly, you can't use
the R_DWTXTADDR_U1 reloc flavor for indirect text references since the
real function count for the package (asm + go) exceeds 255.

The new strategy (with this patch) is to have the compiler look at the
"symabis" file to determine the count of assembly functions. For the
assembler, rather than create additional plumbing to pass in the Go
source func count we just use an dummy (artificially high) function
count so as to select a relocation that will be large enough.

Fixes #72810.
Updates #26379.

Change-Id: I98d04f3c6aacca1dafe1f1610c99c77db290d1d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/663235
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/asm/main.go

@@ -49,6 +49,7 @@ func main() {
 	ctxt.Debugpcln = flags.DebugFlags.PCTab
 	ctxt.IsAsm = true
 	ctxt.Pkgpath = *flags.Importpath
+	ctxt.DwTextCount = objabi.DummyDwarfFunctionCountForAssembler()
 	switch *flags.Spectre {
 	default:
 		log.Printf("unknown setting -spectre=%s", *flags.Spectre)