math: consolidate assembly stub implementations

Where assembly functions are just jumps to the Go implementation
put them into a stubs_<arch>.s file. This reduces the number of
files considerably and makes it easier to see what is really
implemented in assembly.

I've also run the stubs files through asmfmt to format them in
a more consistent way.

Eventually we should replace these 'stub' assembly files with
a pure Go implementation now that we have mid-stack inlining
(see #31362).

Change-Id: If5b2022dcc23e1299f1b7ba79884f1b1263d0f7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/173398


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>