Turns out fenix.packages.aarch64-linux.targets.x86_64-*.minimal.rustc
is an x86_64 compiler. The only component that you actually need to pull
from 'targets' for cross compilation is rust-std.
Because x86_64-unknown-linux-gnu is first in the target list, we were
getting x86 rustc and cargo binaries, making it impossible to compile
grapevine on an aarch64 host.
We were also missing aarch64-unknown-linux-gnu in the targets list,
which is used by the default package on a aarch64 host.
It comes with a bunch of new lints (yay!) so I fixed them all so CI will
keep working.
Also apparently something about linking changed because I had to change
the checks for deciding the linker flags for static x86_64 builds to
keep working.
