This is a CL similar to
https://chromium-review.googlesource.com/c/v8/v8/+/3553006, but on Linux
rather than macOS. The goal is to allow builtins to use short builtin
calls without paying a memory cost, by remapping rather than copying
them.

However, while macOS has a system call making this easier, on Linux we
don't have one on most kernels. There is the recently-introduced
mremap(MREMAP_DONTUNMMAP), which is available in 5.7, but only works on
anonymous mappings until 5.13, which is too recent for most Android
devices.

Instead, we open() the file containing the builtins, and mmap() it at
the desired location.

Change-Id: I4524f349948b8f48c4536cf392a1cd179662a6cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3570426Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80022}