At present, PGA_EXECUTABLE is only used by the powerpc mmu_oea64 pmap. The MI layer only accesses this flag to assert that it is clear when a managed page is freed.
Soon, we will need a similar, but not identical, machine-dependent flag in the arm64 pmap.
So, we rename the flag to PGA_PMAP_PRIV1, simply saying that it is reserved for use by the pmap. Each pmap can then define a name that best reflects its own meaning. However, we still assert that this flag is clear when a managed page is freed.