Remove the bulk of the NaCl port.

Remove the bulk of the NaCl port.

The NaCl port has not been actively maintained since before the 2.25
release.  The complementary GCC back-end was never contributed to GCC,
and we are given to understand that the current NaCl SDK has switched
to Clang and therefore cannot be used to build glibc anymore, so we
doubt that the port remains useful.

This commit simply removes the sysdeps/arm/nacl and sysdeps/nacl
directories and the abi-tags entry.

	Remove the NaCl port.
	* abi-tags: Remove .*-.*-nacl.* entry.
	* sysdeps/arm/nacl: Remove directory and contents.
	* sysdeps/nacl: Likewise.