cmd/link: build shstrtab from ELF sections

cmd/link: build shstrtab from ELF sections

Since before Go 1 the Go linker has handled ELF by first building the
ELF section string table, and then pointing ELF section headers to it.
This duplicates code as sections are effectively created twice,
once with the name and then again with the full section header.
The code duplication also means that it's easy to create unnecessary
section names; for example, every internally linked Go program
currently contains the string ".go.fuzzcntrs" although most do not
have a section by that name.

This CL changes the linker to simply build the section string table
after all the sections are known.

Change-Id: I27ba15b2af3dc1b8d7436b6c409f818aa8e6bfb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/718840
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>