cmd/compile/internal-abi: use x87 mode, not MMX mode

Florian Weimer pointed out that my justification for using MMX mode
was nonsense and that staying in x87 mode simplifies transitions to
and from C. Hence, switch the spec to say we're always in x87 mode.

For #40724.

Change-Id: Iad916b2c376db41f95614aa6897f6b1184576bb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/295789


Trust: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>