[release-branch.go1.15] cmd/internal/goobj2: fix buglet in object file reader

The code in the Go object file reader was casting a pointer to mmaped
memory into a large array prior to performing a read of the
relocations section:

	return (*[1<<20]Reloc)(unsafe.Pointer(&r.b[off]))[:n:n]

For very large object files, this artificial array isn't large enough
(that is, there are more than 1048576 relocs to read), so update the
code to use a larger artifical array size.

Fixes #43214.
Updates #41621.

Change-Id: Ic047c8aef4f8a3839f2e7e3594bce652ebd6bd5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/278492


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Trust: Than McIntosh <thanm@google.com>
(cherry picked from commit f4e7a6b9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/278673