The assembly in math/big may contain instructions that the bootstrap
compiler does not support. Disable it using the math_big_pure_go
build tag.

Fixes #17484.

Change-Id: I766cab6a888721ab4ed76ebdbfc87ad4e919ec41
Reviewed-on: https://go-review.googlesource.com/31142
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>