x86-64: Remove sysdeps/x86_64/x32/dl-machine.h

Remove sysdeps/x86_64/x32/dl-machine.h by folding x32 ARCH_LA_PLTENTER,
ARCH_LA_PLTEXIT and RTLD_START into sysdeps/x86_64/dl-machine.h.  There
are no regressions on x86-64 nor x32.  There are no changes in x86-64
_dl_start_user.  On x32, _dl_start_user changes are

 <_dl_start_user>:
 	mov    %eax,%r12d
+	mov    %esp,%r13d
 	mov    (%rsp),%edx
 	mov    %edx,%esi
-	mov    %esp,%r13d
 	and    $0xfffffff0,%esp
 	mov    0x0(%rip),%edi        # <_dl_start_user+0x14>
 	lea    0x8(%r13,%rdx,4),%ecx

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>