Use SipHasher rather than SHA-512 for ISLE manifest.

Fixes #3609. It turns out that `sha2` is a nontrivial dependency for
Cranelift in many contexts, partly because it pulls in a number of other
crates as well.

One option is to remove the hash check under certain circumstances, as
implemented in #3616. However, this is undesirable for other reasons:
having different dependency options in Wasmtime in particular for
crates.io vs. local builds is not really possible, and so either we
still have the higher build cost in Wasmtime, or we turn off the checks
by default, which goes against the original intent of ensuring developer
safety (no mysterious stale-source bugs).

This PR uses `SipHash` instead, which is built into the standard
library. `SipHash` is deprecated, but it's fixed and deterministic
(across runs and across Rust versions), which is what we need, unlike
the suggested replacement `std::collections::hash_map::DefaultHasher`.
The result is only 64 bits, and is not cryptographically secure, but we
never needed that; we just need a simple check to indicate when we
forget a `rebuild-isle`.

cranelift/codegen/src/isa/x64/lower/isle/generated_code.manifest

@@ -1,4 +1,4 @@
-src/clif.isle be1359b4b6b153f378517c1dd95cd80f4a6bed0c7b86eaba11c088fd71b7bfe80a3c868ace245b2da0bfbbd6ded262ea9576c8e0eeacbf89d03c34a17a709602
-src/prelude.isle 15c8dd937171bd0f619179e219422d43af0eb0ef9a6e88f23b2aa55776712e27342309dd3a4441876b2dfec7f16ce7fe13b3a926ace89b25cfc9577e7b1d1578
-src/isa/x64/inst.isle 1a44ccc0c2cad90447762848461fcae714216ef058d42bdba89330a6008061526e92bbf1c17055c465b20fc75d98d1faa34feda8b22fa7ae0504a0f808798b41
-src/isa/x64/lower.isle c7943201b32e9eb9726466e8cc417f7e84c4c4052de31e05ab6e0ad7502a587cf1d7d9835703c4ff5a506390f7a0668741e7f3feaa1edda6396571a425949fc9
+src/clif.isle f176ef3bba99365
+src/prelude.isle babc931e5dc5b4cf
+src/isa/x64/inst.isle fb5d3ac8e68c46d2
+src/isa/x64/lower.isle d39e01add89178d5