Assume MREMAP_DONTUNMAP always available only on Android

Currently we assume that all target builds have the requisite kernel
features (like MREMAP_DONTUNMAP) for userfaultfd GC, on any kernel >=
5.4. However, that's true only for Android targets. Linux targets may
not have those.

Test: Golem (opt-cc arm targets)
Bug: 244737154
Change-Id: Id8c2231d6aff68ec6b7273a896101cf334b0f2c3
1 file changed