Change aarch64_linux module and lse tests to have the same gating

Trying to run testcrate on non-linux aarch64 currently hits a
compilation error. Make this test linux-only, to be consistent with the
`aarch64_linux` module that it depends on.

Additionally, enable the `aarch64_linux` module for `target_arch =
"arm64ec"` to be the same as these tests.

Author: tgross35

testcrate/tests/lse.rs

@@ -1,8 +1,5 @@
 #![feature(decl_macro)] // so we can use pub(super)
-#![cfg(all(
-    any(target_arch = "aarch64", target_arch = "arm64ec"),
-    not(feature = "no-asm")
-))]
+#![cfg(all(target_arch = "aarch64", target_os = "linux", not(feature = "no-asm")))]
 
 /// Translate a byte size to a Rust type.
 macro int_ty {