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.