Fix testing the cranelift-reader test alone (#9372)

This PR is intended to help unblock #9178. That PR uncovered the
consequence that testing the `cranelift-reader` crate alone does not
work on non-x86_64 platforms. One of its tests relies on parsing the
`x86_64` target specification to work, and that only works if the `x86`
feature of the cranelift-codegen crate is enabled. This PR
unconditionally enables this feature when testing.

This was uncovered on #9178 because the set of crates tested in each
shard of our sharded builders depends on the set of crates in the
workspace. When a new crate is added it may shuffle around which crates
are tested in which location. Previously `cranelift-reader` must have
been always tested with a crate that unconditionally enables
`cranelift-codegen/x86` as a feature, but #9178 got unlucky where it
moved to a set that didn't include this, thus exposing the failure.

cranelift/reader/Cargo.toml

@@ -18,3 +18,7 @@ anyhow = { workspace = true, features = ['std'] }
 cranelift-codegen = { workspace = true }
 smallvec = { workspace = true }
 target-lexicon = { workspace = true, features = ['std'] }
+
+[dev-dependencies]
+# Some tests require that the x86_64 target parses for the target specification.
+cranelift-codegen = { workspace = true, features = ['x86'] }