runtime: re-add sanity check for GCCPUFraction
runtime: re-add sanity check for GCCPUFraction

This no longer appears to be reproducible on windows/386. Try putting
it back and we'll see if the builders still don't like it.

Fixes #19319.

Change-Id: Ia47b034e18d0a5a1951125c00542b021aacd5e8d
Reviewed-on: https://go-review.googlesource.com/47936
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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