cmd/internal/obj/x86: make AsmBuf receiver name consistent

Fixes golint receiver name complaints.

We can't go with "a" name as it sometimes is used for obj.Addr args.

Change-Id: I66556f4e3dc42cfaaa4db3ed7772fa6756ea9a9b
Reviewed-on: https://go-review.googlesource.com/104796
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>