lang/rust: Add COMPAT_FREEBSD11 canary

This attempts to provide a nicer error message for the subset of
users who build their own kernels without COMPAT_FREEBSD11 and then
attempt to build lang/rust.  The Rust ecosystem currently uses
pre-ino64 syscalls, so building lang/rust without COMPAT_FREEBSD11
is not going to work.

The error message for this is non-obvious and there is a new bug
for this at least every 1-2 months.  Hopefully this will improve
the situation a little.

Cargo and Gecko ports are similarly affected, so add the pre-build
check to them too.

Reviewed by:	jbeich, mikael.urankar@gmail.com
Tested by:	madpilot (negative case)
Approved by:	gecko (jbeich)
Differential Revision:	https://reviews.freebsd.org/D23100

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