[GWP-ASan] Refactor memory mapping functions

In preparation for Fuchsia support, this CL refactors the memory
mapping functions.

The new functions are as follows:
- for Freeslots and Metadata:
  `void *map(size_t Size, const char *Name) const;`
  `void unmap(void *Ptr, size_t Size) const;`
- for the Pool:
  `void *reservePool(size_t Size);`
  `void commitPool(void *Ptr, size_t Size) const;`
  `void decommitPool(void *Ptr, size_t Size) const;`
  `void unreservePool();`
  Note that those don't need a `Name` parameter as those are fixed per
  function. `{reserve,unreserve}Pool` are not `const` because they will
  modify platform specific class member on Fuchsia.

I added a plethora of `assert()` as the initial code was not enforcing
page alignment for sizes and addresses, which caused problem in the
initial Fuchsia draft. All sizes should now be properly rounded up to
a page.

Differential Revision: https://reviews.llvm.org/D89993