doc: mention s390x in environment section of install-source.html
doc: mention s390x in environment section of install-source.html

Change-Id: I521ba94356165efc3252ca4b3d364a2bbabd4aab
Reviewed-on: https://go-review.googlesource.com/58010
Run-TryBot: Michael Munday <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

polyverse-security/go,golang/go,akutz/go,cloudflare/go,josharian/go,akutz/go,cloudflare/go,Cofyc/go,CAFxX/go,codestation/go,akutz/go,codestation/go,CAFxX/go,codestation/go,codestation/go,josharian/go,nawawi/go,akutz/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,codestation/go,nawawi/go,polyverse-security/go,christopher-henderson/Go,golang/go,codestation/go,CAFxX/go,polyverse-security/go,nawawi/go,polyverse-security/go,josharian/go,codestation/go,codestation/go,golang/go,nawawi/go,Cofyc/go,golang/go,josharian/go,Cofyc/go,josharian/go,josharian/go,akutz/go,polyverse-security/go,Cofyc/go,CAFxX/go,josharian/go,polyverse-security/go,golang/go,cloudflare/go,CAFxX/go,Cofyc/go,CAFxX/go,cloudflare/go,polyverse-security/go,cloudflare/go,golang/go,CAFxX/go,josharian/go,Cofyc/go,akutz/go,cloudflare/go,cloudflare/go,akutz/go,polyverse-security/go,Cofyc/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,Cofyc/go