GNU as: move deprecation message after option parsing

Some cmake test parses the output of the first line of as --version, and
emits an error if it does not contain some expected strings:

Checking whether the ASM compiler is GNU using "--version" did not match
"(GNU assembler)|(GCC)|(Free Software Foundation)"

Emit the deprecation message later, after parsing argv and thus --version.

PR: 246540
Reported by: dch
Sponsored by: The FreeBSD Foundation