kbuild: improve scripts/gcc-version.sh output a bit when called without args

Currently, if you call scripts/gcc-version.sh without arguments it will
generate this output :

$ sh scripts/gcc-version.sh
scripts/gcc-version.sh: line 12: [: =: unary operator expected
scripts/gcc-version.sh: line 16: -E: command not found
scripts/gcc-version.sh: line 17: -E: command not found
0000

Not too pretty. I believe this is an improvement :

$ sh scripts/gcc-version.sh
Error: No compiler specified.
Usage:
        scripts/gcc-version.sh <gcc-command>

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>