MIPS16: Implement `code_readable` function attribute.

MIPS16: Implement `code_readable` function attribute.

Support for __attribute__ ((code_readable)).  Takes up to one argument of
"yes", "no", "pcrel".  This will change the code readability setting for just
that function.  If no argument is supplied, then the setting is 'yes'.

gcc/ChangeLog:

	* config/mips/mips.cc (enum mips_code_readable_setting):New enmu.
	(mips_handle_code_readable_attr):New static function.
	(mips_get_code_readable_attr):New static enum function.
	(mips_set_current_function):Set the code_readable mode.
	(mips_option_override):Same as above.
	* doc/extend.texi:Document code_readable.

gcc/testsuite/ChangeLog:

	* gcc.target/mips/code-readable-attr-1.c: New test.
	* gcc.target/mips/code-readable-attr-2.c: New test.
	* gcc.target/mips/code-readable-attr-3.c: New test.
	* gcc.target/mips/code-readable-attr-4.c: New test.
	* gcc.target/mips/code-readable-attr-5.c: New test.