charset.c (one_utf8_to_cppchar): Correct mask used for 5-byte UTF-8 sequences.

libcpp:
	* charset.c (one_utf8_to_cppchar): Correct mask used for 5-byte
	UTF-8 sequences.

gcc/testsuite:
	* gcc.dg/cpp/utf8-5byte-1.c: New test.

From-SVN: r147073