elf: Align argument of __munmap to page size [BZ #28676]

On Linux/x86-64, for elf/tst-align3, we now get

munmap(0x7f88f9401000, 1126424)         = 0

instead of

munmap(0x7f1615200018, 544768)          = -1 EINVAL (Invalid argument)

Reviewed-by: Florian Weimer <fweimer@redhat.com>