enable debugging infrastructure when using C backend

Thanks to @jacobly0's recent enhancements to the C backend, this stuff
works now.

Author: andrewrk

lib/std/builtin.zig

@@ -868,8 +868,7 @@ pub fn default_panic(msg: []const u8, error_return_trace: ?*StackTrace, ret_addr
 
     // For backends that cannot handle the language features depended on by the
     // default panic handler, we have a simpler panic handler:
-    if (builtin.zig_backend == .stage2_c or
-        builtin.zig_backend == .stage2_wasm or
+    if (builtin.zig_backend == .stage2_wasm or
         builtin.zig_backend == .stage2_arm or
         builtin.zig_backend == .stage2_aarch64 or
         builtin.zig_backend == .stage2_x86_64 or

lib/std/debug.zig

@@ -1360,13 +1360,7 @@ pub const DebugInfo = struct {
     }
 
     pub fn getModuleForAddress(self: *DebugInfo, address: usize) !*ModuleDebugInfo {
-        if (builtin.zig_backend == .stage2_c) {
-            return @as(error{
-                InvalidDebugInfo,
-                MissingDebugInfo,
-                UnsupportedBackend,
-            }, error.UnsupportedBackend);
-        } else if (comptime builtin.target.isDarwin()) {
+        if (comptime builtin.target.isDarwin()) {
             return self.lookupModuleDyld(address);
         } else if (native_os == .windows) {
             return self.lookupModuleWin32(address);
@@ -1380,9 +1374,7 @@ pub const DebugInfo = struct {
     }
 
     pub fn getModuleNameForAddress(self: *DebugInfo, address: usize) ?[]const u8 {
-        if (builtin.zig_backend == .stage2_c) {
-            return null;
-        } else if (comptime builtin.target.isDarwin()) {
+        if (comptime builtin.target.isDarwin()) {
             return null;
         } else if (native_os == .windows) {
             return self.lookupModuleNameWin32(address);
@@ -2191,8 +2183,6 @@ pub fn dumpStackPointerAddr(prefix: []const u8) void {
 }
 
 test "manage resources correctly" {
-    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // error.UnsupportedBackend
-
     if (builtin.os.tag == .wasi) return error.SkipZigTest;
 
     if (builtin.os.tag == .windows and builtin.cpu.arch == .x86_64) {

lib/std/os.zig

@@ -5371,8 +5371,10 @@ pub fn dl_iterate_phdr(
 ) Error!void {
     const Context = @TypeOf(context);
 
-    if (builtin.object_format != .elf)
-        @compileError("dl_iterate_phdr is not available for this target");
+    switch (builtin.object_format) {
+        .elf, .c => {},
+        else => @compileError("dl_iterate_phdr is not available for this target"),
+    }
 
     if (builtin.link_libc) {
         switch (system.dl_iterate_phdr(struct {

src/Module.zig

@@ -6629,7 +6629,8 @@ pub fn backendSupportsFeature(mod: Module, feature: Feature) bool {
             mod.comp.bin_file.options.use_llvm,
         .panic_unwrap_error => mod.comp.bin_file.options.target.ofmt == .c or
             mod.comp.bin_file.options.use_llvm,
-        .safety_check_formatted => mod.comp.bin_file.options.use_llvm,
+        .safety_check_formatted => mod.comp.bin_file.options.target.ofmt == .c or
+            mod.comp.bin_file.options.use_llvm,
         .error_return_trace => mod.comp.bin_file.options.use_llvm,
         .is_named_enum_value => mod.comp.bin_file.options.use_llvm,
         .error_set_has_value => mod.comp.bin_file.options.use_llvm or mod.comp.bin_file.options.target.isWasm(),