[CPU] Remove use_haswell_instructions in favor of x64_extension_mask

Rather than having a single bool to conditionally detect haswell-level
instruction features. The granularity is increased with a new
`x64_extension_mask` where individual features within the x64 backend
can be turned on or off in a bit-mask manner. Since we have an ARM
backend on the horizon, I've added this to the new `x64`
configuration-group rather than `CPU`. This new pattern will hopefully
allow for testing to be more targetted to certain processor features and
allows the user to determine if they want certain features to be enabled
or disabled(such as avoiding BMI2 on certain AMD processors due to
pdep/pext being incredibly slow). The default configuration is to detect
and utilize all available features.

src/xenia/cpu/ppc/testing/ppc_testing_main.cc

@@ -422,8 +422,7 @@ bool RunTests(const std::string_view test_name) {
   int failed_count = 0;
   int passed_count = 0;
 
-  XELOGI("Haswell instruction usage {}.",
-         cvars::use_haswell_instructions ? "enabled" : "disabled");
+  XELOGI("Instruction feature mask {}.", cvars::x64_extension_mask);
 
   auto test_path_root = cvars::test_path;
   std::vector<std::filesystem::path> test_files;