src: `enable_target_feature` should add features, not overwrite

`llvm_features` being empty is the default state, and implies the
presence of certain features.

Previously if any target features were explicitly enabled by the
`enable_target_feature` attribute, they were added comma separated
to `llvm_features`.

For example: `lzcnt,popcnt,...,sse4.2,sse`

This was causing LLVM to try to target a CPU that *ONLY* has the
explicitly enabled features.  This now will prefix explicitly enabled
features with a `+`, and preserve the existing `llvm_features` string
by appending to it if it is set.

src/build_settings.cpp

@@ -1493,7 +1493,7 @@ gb_internal void enable_target_feature(TokenPos pos, String const &target_featur
 }
 
 
-gb_internal char const *target_features_set_to_cstring(gbAllocator allocator, bool with_quotes) {
+gb_internal char const *target_features_set_to_cstring(gbAllocator allocator, bool with_quotes, bool with_plus) {
 	isize len = 0;
 	isize i = 0;
 	for (String const &feature : build_context.target_features_set) {
@@ -1502,6 +1502,7 @@ gb_internal char const *target_features_set_to_cstring(gbAllocator allocator, bo
 		}
 		len += feature.len;
 		if (with_quotes) len += 2;
+		if (with_plus) len += 1;
 		i += 1;
 	}
 	char *features = gb_alloc_array(allocator, char, len+1);
@@ -1513,6 +1514,7 @@ gb_internal char const *target_features_set_to_cstring(gbAllocator allocator, bo
 		}
 
 		if (with_quotes) features[len++] = '"';
+		if (with_plus) features[len++] = '+';
 		gb_memmove(features + len, feature.text, feature.len);
 		len += feature.len;
 		if (with_quotes) features[len++] = '"';

src/llvm_backend.cpp

@@ -2531,7 +2531,46 @@ gb_internal bool lb_generate_code(lbGenerator *gen) {
 	*/
 
 	if (build_context.target_features_set.entries.count != 0) {
-		llvm_features = target_features_set_to_cstring(permanent_allocator(), false);
+		// Prefix all of the features with a `+`, because we are
+		// enabling additional features.
+		char const *additional_features = target_features_set_to_cstring(permanent_allocator(), false, true);
+
+		String f_string = make_string_c(llvm_features);
+		String a_string = make_string_c(additional_features);
+		isize f_len = f_string.len;
+
+		if (f_len == 0) {
+			// The common case is that llvm_features is empty, so
+			// the target_features_set additions can be used as is.
+			llvm_features = additional_features;
+		} else {
+			// The user probably specified `-microarch:native`, so
+			// llvm_features is populated by LLVM's idea of what
+			// the host CPU supports.
+			//
+			// As far as I can tell, (which is barely better than
+			// wild guessing), a bitset is formed by parsing the
+			// string left to right.
+			//
+			// So, llvm_features + ',' + additonal_features, will
+			// makes the target_features_set override llvm_features.
+
+			char *tmp = gb_alloc_array(permanent_allocator(), char, f_len + 1 + a_string.len + 1);
+			isize len = 0;
+
+			// tmp = f_string
+			gb_memmove(tmp, f_string.text, f_string.len);
+			len += f_string.len;
+			// tmp += ','
+			tmp[len++] = ',';
+			// tmp += a_string
+			gb_memmove(tmp + len, a_string.text, a_string.len);
+			len += a_string.len;
+			// tmp += NUL
+			tmp[len++] = 0;
+
+			llvm_features = tmp;
+		}
 	}
 
 	// GB_ASSERT_MSG(LLVMTargetHasAsmBackend(target));