This function tries to determine the number of virtual address bits
available on the current CPU and with that the maximum size of the
userspace address space. On x64, it can be implemented through CPUID.

The result of this function is now used in two ways: first, it limits
the maximum size of the virtual memory cage, currently to a quarter of
the address space. Second, it influences the placement of fake cages,
which are attempted to be placed into the lower half of the address
space so that they are followed by large amounts of (hopefully) unused
but addressable virtual memory in which pages can be allocated.

Bug: chromium:1218005
Change-Id: I0edc5d241d899f16dbc47492fa1534b6aaa4aa13
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3220348
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77393}