std.posix.abort: only trigger breakpoint on Windows if being debugged

Processes should reasonably be able to expect their children to abort
with typical exit codes, rather than a debugger breakpoint signal. This
flag in the PEB is what would be checked by `IsDebuggerPresent` in
kernel32, which is the function you would typically use for this
purpose.

This fixes `test-stack-trace` failures on Windows, as these tests were
expecting exit code 3 to indicate abort.

lib/std/posix.zig

@@ -689,7 +689,7 @@ pub fn abort() noreturn {
     // even when linking libc on Windows we use our own abort implementation.
     // See https://github.com/ziglang/zig/issues/2071 for more details.
     if (native_os == .windows) {
-        if (builtin.mode == .Debug) {
+        if (builtin.mode == .Debug and windows.peb().BeingDebugged != 0) {
             @breakpoint();
         }
         windows.kernel32.ExitProcess(3);