[libc++][TZDB] Fixes relative path resolving. (#87882)

The path /etc/localtime is a symlink. This symlink can be a relative
path. This fixes resolving a relative symlink.

Since the path used is hard-coded based on the user's system there is no
good way to test this.

Fixes: https://github.com/llvm/llvm-project/issues/87872