[Sanitizer] Fix page alignment for mmap calls

We are in the process of enabling sanitizer_common unit tests on arm64 for apple devices. rdar://101436019

The test `CompactRingBuffer.int64` is failing on arm64 with the error:

```==17265==ERROR: SanitizerTool failed to deallocate 0xfffffffffffff000 (-4096) bytes at address 0x000105c30000
SanitizerTool: CHECK failed: sanitizer_posix.cpp:63 "(("unable to unmap" && 0)) != (0)" (0x0, 0x0) (tid=157296)```

If page size is sufficiently larger than alignment then this code:
   UnmapOrDie((void*)end, map_end - end);
end is will be greater than map_end causing the value passed to UnmapOrDie to be negative.

This is caused when GetPageSizeCached returns 16k and alignment is 8k.
map_size and what is mapped by mmap uses size and alignment which is smaller than what is calculated by end using the actual page size.
Therefore, map_end ends up being less than end.
The call to mmap is allocating sufficent page-aligned memory, because it calls RoundUp within MmapOrDieOnFatalError.
But this size is not being captured by map_size.

We can address this by rounding up map_size here to be page-aligned. This ensures that map_end will be greater than or equal to end and that it will match mmaps use of page-aligned value, and the
subsequent call to munmap will also be page-aligned.

Differential Revision: https://reviews.llvm.org/D140353