cmd/internal/obj: rename some local variables not c

cmd/internal/obj: rename some local variables not c

I plan to use c as a consistent local variable
in this packages. Rename most variables named c,
excepting only some simple functions in asm9.go.

Changes prepared with gorename.

Passes toolstash-check -all.

Updates #15756

Change-Id: If79baac43fca68fad1076e1ff23ae87c2ba638e4
Reviewed-on: https://go-review.googlesource.com/40172
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>