Allow PROT_EXEC on the zygote mappings when falling back to ashmem

Because we now protect the mappings before creating the executable one,
we need to also allow PROT_EXEC on an ashmem region.

Test: manually change code to use ashmem, build, see that JIT is
created.
Bug: 202999239

Merged-In: I9f2944c45f4ff607c7e341552c82138bfe13cca4
(cherry picked from commit 8c63ce27d4d3e87f535769976e9b9185fa372b9b)
Change-Id: I8a5e9655e6c40404389344e78c21b0ebd5a1a4b9