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.

cranelift/codegen/src/settings.rs

@@ -541,6 +541,7 @@ enable_atomics = true
 enable_safepoints = false
 enable_llvm_abi_extensions = false
 unwind_info = true
+preserve_frame_pointers = false
 machine_code_cfg_info = false
 emit_all_ones_funcaddrs = false
 enable_probestack = true