default_panic: handle ".other" OS like ".freestanding"

The default panic handler needs to handle .other like .freestanding (i.e.,
infinite loop), as it cannot expect any of the std.debug.panicImpl code to
work (which brings in Locks, Threads and Progress, etc).

Also, update the unwind_freestanding.zig test case to also run on the
.other OS target, too.  This should keep the stacktrace generation in
debug.zig working with that target.

Author: rootbeer

lib/std/builtin.zig

@@ -781,7 +781,7 @@ pub fn default_panic(msg: []const u8, error_return_trace: ?*StackTrace, ret_addr
     }
 
     switch (builtin.os.tag) {
-        .freestanding => {
+        .freestanding, .other => {
             while (true) {
                 @breakpoint();
             }

test/standalone/stack_iterator/build.zig

@@ -98,13 +98,14 @@ pub fn build(b: *std.Build) void {
     // Unwinding without libc/posix
     //
     // No "getcontext" or "ucontext_t"
-    {
+    const no_os_targets = [_]std.Target.Os.Tag{ .freestanding, .other };
+    inline for (no_os_targets) |os_tag| {
         const exe = b.addExecutable(.{
             .name = "unwind_freestanding",
             .root_source_file = b.path("unwind_freestanding.zig"),
             .target = b.resolveTargetQuery(.{
                 .cpu_arch = .x86_64,
-                .os_tag = .freestanding,
+                .os_tag = os_tag,
             }),
             .optimize = optimize,
             .unwind_tables = null,

test/standalone/stack_iterator/unwind_freestanding.zig

@@ -36,7 +36,7 @@ noinline fn frame0(expected: *[4]usize, unwound: *[4]usize) void {
     frame1(expected, unwound);
 }
 
-// Freestanding entrypoint
+// No-OS entrypoint
 export fn _start() callconv(.C) noreturn {
     var expected: [4]usize = undefined;
     var unwound: [4]usize = undefined;
@@ -50,8 +50,9 @@ export fn _start() callconv(.C) noreturn {
         }
     }
 
-    // Need to compile as "freestanding" to exercise the StackIterator code, but when run as a
-    // regression test need to actually exit.  So assume we're running on x86_64-linux ...
+    // Need to compile with the target OS as "freestanding" or "other" to
+    // exercise the StackIterator code, but when run as a regression test
+    // need to actually exit.  So assume we're running on x86_64-linux ...
     asm volatile (
         \\movl $60, %%eax
         \\syscall