cmd/internal/goobj: fix buglet in object file reader

The code in the new (introduced in 1.15) Go object file reader was
casting a pointer-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 #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>