Revert "std.Target: xtensa defaults to windowed ABI"

This reverts commit c55e83e.

This was a misreading of XtensaFeatures.td on my part.

Author: alexrp

lib/std/Target.zig

@@ -3713,7 +3713,7 @@ pub fn cCallingConvention(target: *const Target) ?std.builtin.CallingConvention
         .sh, .sheb => .{ .sh_gnu = .{} },
         .ve => .{ .ve_sysv = .{} },
         .xcore => .{ .xcore_xs1 = .{} },
-        .xtensa, .xtensaeb => .{ .xtensa_windowed = .{} },
+        .xtensa, .xtensaeb => .{ .xtensa_call0 = .{} },
         .amdgcn => .{ .amdgcn_device = .{} },
         .nvptx, .nvptx64 => .nvptx_device,
         .spirv32, .spirv64 => .spirv_device,