cmd/compile: update maxPtrmaskBytes comment for larger value
cmd/compile: update maxPtrmaskBytes comment for larger value

The comment for maxPtrmaskBytes implied that the value was still 16,
but that changed in CL 10815.

Change-Id: I86e304bc7d9d1a0a6b22b600fefcc1325e4372d9
Reviewed-on: https://go-review.googlesource.com/36120
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

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