The syscall package currently declares RawSyscall6 for every GOOS, but
does not define it on Solaris. This leads to code using said function
to compile but it will not link. Fix it by adding RawSyscall6 and make
it panic.

Also remove the obsolete comment above runtime.syscall_syscall as
pointed out by Aram.

Updates #24357

Change-Id: I1b1423121d1c99de2ecc61cd9a935dba9b39e3a4
Reviewed-on: https://go-review.googlesource.com/100655Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>