cmd/link: issue error if elf header overruns

cmd/link: issue error if elf header overruns

This is probably unlikely in practice, but when debugging alignment
related issues on ppc64 using very small text section splits, the elf
header could grow beyond the preallocated space and quietly stomp
on the first few text sections.

Change-Id: Ided58aa0b1e60f9da4b3cb277e4ebafcee4ec693
Reviewed-on: https://go-review.googlesource.com/c/go/+/307430
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>