Move spec interpreter fuzzing behind a Cargo feature (#3871)

* Move spec interpreter fuzzing behind a Cargo feature

Building the spec interpreter requires a local installation of Ocaml and
now libgmp which isn't always available, so this enables the ability to
disable building the spec interpreter by using `cargo +nightly fuzz
build --no-default-features`. The spec interpreter is still built by
default but if fuzzers are being built locally and the spec interpreter
isn't needed then this should enable it to be relatively easily
opted-out of.

* Tweak manifest directives

fuzz/Cargo.toml

@@ -21,10 +21,8 @@ wasmtime = { path = "../crates/wasmtime" }
 wasmtime-fuzzing = { path = "../crates/fuzzing" }
 
 [features]
-# Leave a stub feature with no side-effects in place for now: the OSS-Fuzz
-# config builds fuzz targets with this feature enabled and we don't want to
-# break the build.
-experimental_x64 = []
+default = ['fuzz-spec-interpreter']
+fuzz-spec-interpreter = ['wasmtime-fuzzing/fuzz-spec-interpreter']
 
 [[bin]]
 name = "compile"
@@ -61,6 +59,7 @@ name = "differential_spec"
 path = "fuzz_targets/differential_spec.rs"
 test = false
 doc = false
+required-features = ['fuzz-spec-interpreter']
 
 [[bin]]
 name = "differential_wasmi"