Lavapipe support on Fuchsia.

This primarily involves adding BUILD.gn files corresponding to the
existing meson.build files.  However, some other changes were needed,
most notably providing LLVM with a custom memory mapper which is able
to mark allocated pages as executable (Fuchsia does this differently
than other platforms).

Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/976872
Reviewed-by: John Rosasco <rosasco@google.com>

[lavapipe] Fix build error, missing input

Change-Id: I8870024328c59072c945e79d672dd17a2e4b3797
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1005629
Reviewed-by: John Bauman <jbauman@google.com>
Commit-Queue: John Rosasco <rosasco@google.com>
24 files changed