mini-std: Add -frust-mini-std flag

[CohenArthur]

gcc/rust/ChangeLog:

* lang.opt: Add flag to lang options.

This flag should be used to enable the inclusion of @P-E-P's mini-std library, which is being worked on in #2957

[CohenArthur]

Help message looks better like this :) Will you also add the inclusion mechanism in this PR ?

Considering the inclusion mechanism depends on your PR, I was thinking of doing that after :)