S390: Fix utf32 to utf16 handling of low surrogates (disable cu42).

According to the latest Unicode standard, a conversion from/to UTF-xx has
to report an error if the character value is in range of an utf16 surrogate
(0xd800..0xdfff). See https://sourceware.org/ml/libc-help/2015-12/msg00015.html.

Thus the cu42 instruction, which converts from utf32 to utf16,  has to be
disabled because it does not report an error in case of a value in range of
a low surrogate (0xdc00..0xdfff). The etf3eh variant is removed and the c,
vector variant is adjusted to handle the value in range of an utf16 low
surrogate correctly.

ChangeLog:

	* sysdeps/s390/utf16-utf32-z9.c: Disable cu42 instruction and report
	an error in case of a value in range of an utf16 low surrogate.