gnu-smalltalk: generational GC requires libsigsegv

In 64 bit, --without-libsigsegv implies --disable-generational-gc

The build was failing for me, now at least it builds by disabling
libsigsegv it seems to work (on small programs)

Closes #17576.

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