S390: Optimize utf8-utf16 module.

S390: Optimize utf8-utf16 module.

This patch reworks the s390 specific module to convert between utf8 and utf16.
Now ifunc is used to choose either the c or etf3eh (with convert utf instruction)
variants at runtime. Furthermore a new vector variant for z13 is introduced
which will be build and chosen if vector support is available at build / runtime.

In case of converting utf 8 to utf16, the vector variant optimizes input of
1byte utf8 characters. The convert utf instruction is used if a multibyte utf8
character is found.

For the other direction utf16 to utf8, the cu21 instruction can't be re-enabled,
because it does not report an error, if the input-stream consists of a single
low surrogate utf16 char (e.g. 0xdc00). This applies to the newest z13, too.
Thus there is only the c or the new vector variant, which can handle 1..4 byte
utf8 characters.

The c variant from utf16 to utf8 has beed fixed. If a high surrogate was at the
end of the input-buffer, then errno was set to EINVAL and the input-pointer
pointed just after the high surrogate. Now it points to the beginning of the
high surrogate.

This patch also fixes some whitespace errors. The c variant from utf8 to utf16
is now checking that tail-bytes starts with 0b10... and the value is not in
range of an utf16 surrogate.

Furthermore, the etf3eh variants are handling the "UTF-xx//IGNORE" case now.
Before they ignored the ignore-case and always stopped at an error.

ChangeLog:

	* sysdeps/s390/s390-64/utf8-utf16-z9.c: Use ifunc to select c,
	etf3eh or new vector loop-variant.