Fix up default header paths for CloudABI.

CloudABI is a pure cross compilation target. This means that we should
not add /usr/include and /usr/local/include. Instead, headers are stored
in $sysroot/$triple/include.

The method of going back to the sysroot (by using "../../..") is also
used in this function for some of the other environments (e.g., MinGW).

llvm-svn: 231913