math: use s390x mnemonics rather than binary encodings

TMLL, LGDR and LDGR have all been added to the Go assembler
previously, so we don't need to encode them using WORD and BYTE
directives anymore. This is purely a cosmetic change, it does not
change the contents of any object files.

Change-Id: I93f815b91be310858297d8a0dc9e6d8e3f09dd65
Reviewed-on: https://go-review.googlesource.com/129895


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