fiber: make madvise(2) arguments page aligned with ASAN slab cache

Regularly fiber stack slab is page aligned. So upper stack border is
page aligned too when stack grows down. But with ASAN friendly slab
cache implementation this border is not page aligned. As a result
madvise call on stack may zero memory beyond stack slab which will cause
heap corruption. In debug build corruption is detected by assertion:

NO_WRAP
 >  Fatal glibc error: malloc.c:2593 (sysmalloc): assertion failed: (old_top
 >  == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >=
 >  MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize
 >  - 1)) == 0)
NO_WRAP

Interestingly enough the issue can not be investigated using ASAN. The
memory is zeroed by kernel code which is not instrumented so it is
invisible for sanitizer.

Looks like non-ASAN builds are not affected. Even if stack_size is
not page aligned the slab allocated for stack is page aligned. Thus
memory zeroing will be inside the slab and there will be no memory
corruption.

Also when stack grows up lower stack border in not aligned even with
regular small implementation. So madvise call will fail with EINVAL as
it is required that start address is page aligned. We ignore the error
though. Let's fix this issue too while we at it.

Let's introduce fiber_madvise_aligned to align madvise range with proper
direction before calling madvise(2). To justify its usage note that
besides fixing the issues described above, in case of stack growing down
fiber->stack is page aligned and in case of stack growing up
fiber->stack + fiber->stack_size is page aligned.

Part of #7327

NO_TEST=tested by ASAN (debug build)
NO_CHANGELOG=has effect only with newly introduced ASAN friendly slab cache
NO_DOC=has effect only with newly introduced ASAN friendly slab cache

(cherry picked from commit 130c7807)