[dev.link] cmd/compile, cmd/asm: add length to hashed symbols

[dev.link] cmd/compile, cmd/asm: add length to hashed symbols

While working on deduplicating pcdata, I found that the following hashed
symbols would result in the same:

[] == [0,0,0,0....]

This makes using content addressable symbols untenable for pcdata.
Adding the length to the hash keeps the dream alive.

No difference in binary size (darwin, cmd/compile), spurious
improvements in DWARF phase memory.

Change-Id: I21101f7754a3d870922b0dea39c947cc8509432f
Reviewed-on: https://go-review.googlesource.com/c/go/+/247903
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>