cranelift: Add a flag for preserving frame pointers (#4469)

Preserving frame pointers -- even inside leaf functions -- makes it easy to
capture the stack of a running program, without requiring any side tables or
metadata (like `.eh_frame` sections). Many sampling profilers and similar tools
walk frame pointers to capture stacks. Enabling this option will play nice with
those tools.

crates/wasmtime/src/engine.rs

@@ -393,6 +393,7 @@ impl Engine {
             | "machine_code_cfg_info"
             | "tls_model" // wasmtime doesn't use tls right now
             | "opt_level" // opt level doesn't change semantics
+            | "preserve_frame_pointers" // we don't currently rely on frame pointers
             | "enable_alias_analysis" // alias analysis-based opts don't change semantics
             | "probestack_func_adjusts_sp" // probestack above asserted disabled
             | "probestack_size_log2" // probestack above asserted disabled