MFC 233670,233671:

MFC 233670,233671:
- Use VM_MEMATTR_UNCACHEABLE for the constant for UC memory rather than
  VM_MEMATTR_UNCACHED on mips.
- Rename VM_MEMATTR_UNCACHED to VM_MEMATTR_WEAK_UNCACHEABLE on x86 to
  be less ambiguous and more clearly identify what it means.  An alias
  from VM_MEMATTR_WEAK_UNCACHEABLE to VM_MEMATTR_WEAK_UNCACHED remains
  on x86 to preserve the KPI.
- Remove the VM_MEMATTR_UNCACHED alias from powerpc.

git-svn-id: svn://svn.freebsd.org/base/stable/9@234179 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f