Rename <xlocale.h> back to <sys/_locale.h>

libX11 provides <X11/Xlocale.h>.  The build of libX11 itself adds
include/X11 to the compiler's include path.  This results in a name
collision with /usr/include/xlocale.h on case-insensitive filesystems.

Commit 90e35b1eb3 renamed sys/_locale.h to xlocale.h in March 2017 under
the assumption that we should provide the locale_t type in the same file
as on Linux, FreeBSD, and Darwin.

A few weeks later (June 2017), glibc removed the xlocale.h file in favor
of bits/types/locale_t.h, which shouldn't be included directly anyway.

For reference and the reasoning, see
https://sourceware.org/git/?p=glibc.git;a=commit;h=f0be25b6336d

Given the above, revert 90e35b1eb3 and
fix additional usage of xlocale.h.

newlib/libc/include/inttypes.h

@@ -22,7 +22,7 @@
 #include <stddef.h>
 
 #if __BSD_VISIBLE
-#include <xlocale.h>
+#include <sys/_locale.h>
 #endif
 
 #define __STRINGIFY(a) #a