cmd/asm,runtime: reduce spellings of GOEXPERIMENTs

Currently, the objabi.Experiment fields use Go-standard CamelCase, the
GOEXPERIMENT environment variable flags and build tags use all
lowercase, and the asm macros use upper-case with underscores.

This CL makes asm use the lowercase names for macros so there is one
less spelling, e.g., GOEXPERIMENT_regabiargs. This also makes them
consistent with the GOOS_* and GOARCH_* macros, which also use lower
case.

Change-Id: I305cd89af5e8cd1a89cc148746c034bcfd76db3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/307816


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>