runtime: extract Linux auxv handling
runtime: extract Linux auxv handling

This refactoring is in preparation for handling auxv differently in
Android shared libraries.

Updates #18041.

Change-Id: If0458a309f9c804e7abd0a58b5a224d89f8da257
Reviewed-on: https://go-review.googlesource.com/33651
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

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