ruby: Pass --without-gmp unless build.with? "gmp"

Ruby will pick up gmp if it's on your machine, unless it's specifically
told to not look for gmp.  This is true regardless of whether the user
invokes the Homebrew flag --with-gmp.  It's safest to force ruby to not
look for gmp unless the user has specifically requested gmp with the
Homebrew option.  Otherwise, if the user brews ruby with gmp already on
the machine, then later removes gmp, then ruby is broken.

Closes #27795.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>