runtime: improve systemstack-on-Go stack message
runtime: improve systemstack-on-Go stack message

We reused the old C stack check mechanism for the implementation of
//go:systemstack, so when we execute a //go:systemstack function on a
user stack, the system fails by calling morestackc. However,
morestackc's message still talks about "executing C code".

Fix morestackc's message to reflect its modern usage.

Change-Id: I7e70e7980eab761c0520f675d3ce89486496030f
Reviewed-on: https://go-review.googlesource.com/38572
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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