[Support] Don't check MAP_ANONYMOUS, just use MAP_ANON

Though being marked "deprecated" by the Linux man-pages project
(MAP_ANON is a synonym of MAP_ANONYMOUS), it is the mostly widely
available macro - many systems that don't provide MAP_ANONYMOUS have
MAP_ANON. MAP_ANON is also used here and there in compiler-rt.

llvm-svn: 359758