vm_page: Replace PGA_EXECUTABLE with PGA_MACHDEP0004

[alc]

Summary

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_MACHDEP0004, 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.

Test Plan

No functional change.

[kib]

I would not encode the bit position in the name, for instance how it was done for OBJ_PAGEPRIV1/2.