cmd/dist: disable math/big assembly when using the bootstrap compiler
cmd/dist: disable math/big assembly when using the bootstrap compiler

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 <8f145917441a88646319703b14b4bef730f48765@ca.ibm.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

christopher-henderson/Go,vsekhar/elastic-go,suncycheng/go,CAFxX/go,derekmarcotte/go,nawawi/go,linux-on-ibm-z/go,ImJasonH/go,golang/go,CAFxX/go,codestation/go,derekmarcotte/go,goodlang/good,CAFxX/go,cloudflare/go,akutz/go,linux-on-ibm-z/go,codestation/go,derekmarcotte/go,vsekhar/elastic-go,momchil-velikov/go,CAFxX/go,josharian/go,ImJasonH/go,golang/go,ImJasonH/go,josharian/go,nawawi/go,linux-on-ibm-z/go,codestation/go,goodlang/good,derekmarcotte/go,linux-on-ibm-z/go,ImJasonH/go,codestation/go,josharian/go,derekmarcotte/go,Cofyc/go,akutz/go,codestation/go,suncycheng/go,golang/go,goodlang/good,polyverse-security/go,goodlang/good,akutz/go,derekmarcotte/go,derekmarcotte/go,ImJasonH/go,codestation/go,polyverse-security/go,cloudflare/go,suncycheng/go,suncycheng/go,vsekhar/elastic-go,suncycheng/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,akutz/go,goodlang/good,vsekhar/elastic-go,momchil-velikov/go,cloudflare/go,cloudflare/go,josharian/go,CAFxX/go,Cofyc/go,Cofyc/go,nawawi/go,CAFxX/go,momchil-velikov/go,nawawi/go,suncycheng/go,Cofyc/go,cloudflare/go,momchil-velikov/go,Cofyc/go,goodlang/good,ImJasonH/go,derekmarcotte/go,nawawi/go,Cofyc/go,ImJasonH/go,polyverse-security/go,polyverse-security/go,linux-on-ibm-z/go,cloudflare/go,goodlang/good,josharian/go,polyverse-security/go,golang/go,goodlang/good,suncycheng/go,akutz/go,cloudflare/go,polyverse-security/go,Cofyc/go,golang/go,CAFxX/go,golang/go,nawawi/go,codestation/go,josharian/go,polyverse-security/go,josharian/go,momchil-velikov/go,akutz/go,Cofyc/go,nawawi/go,golang/go,momchil-velikov/go,ImJasonH/go,josharian/go,vsekhar/elastic-go,vsekhar/elastic-go,suncycheng/go,polyverse-security/go,codestation/go,momchil-velikov/go,momchil-velikov/go,akutz/go,linux-on-ibm-z/go,linux-on-ibm-z/go,nawawi/go,vsekhar/elastic-go,vsekhar/elastic-go,linux-on-ibm-z/go