rtld: Round down relro_size

rtld: Round down relro_size

lld rounds up p_memsz(PT_GNU_RELRO) to satisfy common-page-size. If the
page size is smaller than common-page-size, rounding up relro_size may
incorrectly make some RW pages read-only.

GNU ld, gold, and ld.lld ensures p_vaddr+p_memsz is a multiple of
common-page-size. While max-page-size >= system the page size,
common-page-size can be smaller than the system page size.

Submitted by:	MaskRay
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D31498