Mk/Uses/cargo.mk: Enable WITH_LTO by default

Ports utilizing Cargo will now enable Link Time Optimization (LTO),
set error handling mechanism to abort and codegen-units to 1 by default.
Exceptions are powerpc64 and riscv64 where LTO is known to be broken
and disabled to avoid unnecessary breakage. Ports utilizing LTO will
output "Additional optimization to port applied" during build.
WITH_LTO is handled by Mk/Features/lto.mk and to disable define
LTO_UNSAFE=yes.

I'd also like to mention arrowd and tobik for helpful input
and thanks to everyone involved.

Reviewed by: mikael (via irc), pkubaj
Tested by: eduardo (build)
Approved by: rust (mikael, pkubaj)
Differential Revision: https://reviews.freebsd.org/D36736