Add a setting to choose a register allocator algorithm to use with MachBackend;

cranelift/codegen/src/settings.rs

@@ -377,27 +377,29 @@ mod tests {
         let f = Flags::new(b);
         assert_eq!(
             f.to_string(),
-            "[shared]\n\
-             opt_level = \"none\"\n\
-             tls_model = \"none\"\n\
-             libcall_call_conv = \"isa_default\"\n\
-             baldrdash_prologue_words = 0\n\
-             probestack_size_log2 = 12\n\
-             enable_verifier = true\n\
-             is_pic = false\n\
-             use_colocated_libcalls = false\n\
-             avoid_div_traps = false\n\
-             enable_float = true\n\
-             enable_nan_canonicalization = false\n\
-             enable_pinned_reg = false\n\
-             use_pinned_reg_as_heap_base = false\n\
-             enable_simd = false\n\
-             enable_atomics = true\n\
-             enable_safepoints = false\n\
-             emit_all_ones_funcaddrs = false\n\
-             enable_probestack = true\n\
-             probestack_func_adjusts_sp = false\n\
-             enable_jump_tables = true\n"
+            r#"[shared]
+regalloc = "backtracking"
+opt_level = "none"
+tls_model = "none"
+libcall_call_conv = "isa_default"
+baldrdash_prologue_words = 0
+probestack_size_log2 = 12
+enable_verifier = true
+is_pic = false
+use_colocated_libcalls = false
+avoid_div_traps = false
+enable_float = true
+enable_nan_canonicalization = false
+enable_pinned_reg = false
+use_pinned_reg_as_heap_base = false
+enable_simd = false
+enable_atomics = true
+enable_safepoints = false
+emit_all_ones_funcaddrs = false
+enable_probestack = true
+probestack_func_adjusts_sp = false
+enable_jump_tables = true
+"#
         );
         assert_eq!(f.opt_level(), super::OptLevel::None);
         assert_eq!(f.enable_simd(), false);