cmd/link: mark PIE binaries as ET_DYN
cmd/link: mark PIE binaries as ET_DYN

Part of adding PIE internal linking on linux/amd64.

Change-Id: I586e7c2afba349281168df5e20d2fdcb697f6e37
Reviewed-on: https://go-review.googlesource.com/28542
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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