cmd/link: extract windows-specific code from dynrelocsym
cmd/link: extract windows-specific code from dynrelocsym

No functional changes.

Change-Id: Ib31bb3f01b515aac6428ec61e0ef02b269623890
Reviewed-on: https://go-review.googlesource.com/62470
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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