cmake: Add option to pass flags not used for host tools

[jbruechert]

This allows to target machines that support more instructions than the build machine.

Not sure if this is a good way to achieve this, but it at least doesn't require significant changes.

[felixguendling]

Compiling mimalloc with -Weverything [...] -Werror has a good chance to cause trouble because we don't want to fix random warnings in mimalloc when we update. So maybe it would be better to have a separate set of flags (MOTIS_ARCH_FLAGS?) that's independent of the aggressive -Weverything [...] -Werror flags that are only relevant for our own code.

[jbruechert]

That's pretty mich what this does except for different variable naming, isn't it?

[felixguendling]

Ahh. Mistook it for motis-compile-options.