Add mremap to std.posix, and use it in std.heap.PageAllocator when available.

[SeanTheGleaming]

Additionally moves std.heap.next_mmap_addr_hint into std.heap.PageAllocator, the only place in the stdlib where it is used.
As it stands, this PR is fully functional, but I am setting this as a draft, as between its limited support and sometimes variable length arguments, there is some ambiguity around the proper way to expose mremap in std.posix. My hope is to get the opinions of more prominent contributors and potentially core members.
I haven't done any serious or proper benchmarks, but on targets which support mremap, just from watching the output of naive test programs, this seems to have made a substantial improvement in the performance of resize operations in std.heap.page_allocator.

[SeanTheGleaming]

It should be mentioned that this PR also resolved some TODOs in PageAllocator.zig

[alexrp]

Just noting that this is very likely to have merge conflicts with #20511.