syscall: clean up mkasm related changes

The mkasm_darwin.go file was renamed to mkasm.go in CL 270380, with OpenBSD
support being added. The mkasm_openbsd.go file should not have been merged,
so remove it. Fix up references to mkasm_$GOOS.go and provide $GOOS as an
argument on invocation.

Updates #36435

Change-Id: I868d3f2146973d026e6a663d437749dbb6b312ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/286812


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>