[libc++][AIX] Do not assert chmod return value is non-zero.

A number of the filesystem tests create a directory that contains a bad
symlink. On AIX recursively setting permissions on said directory will
return a non-zero value because of the bad symlink, however the
following rm -r still completes successfully. Avoid the assertion on
AIX, and rely on the return value of the remove command to detect
problems.

Differential Revision: https://reviews.llvm.org/D112086