Add BionicAllocator::memalign

Bionic needs this functionality to allocate a TLS segment with greater
than 16-byte alignment. For simplicity, this allocator only supports up
to one page of alignment.

The memory layout changes slightly when allocating an object of exactly
PAGE_SIZE alignment. Instead of allocating the page_info header at the
start of the page containing the pointer, it is allocated at the start
of the preceding page.

Bug: http://b/78026329
Test: linker-unit-tests{32,64}
Change-Id: I1c8d1cd7ca72d113bced5ee15ba8d831426b0081