Backport of Ole John's proposed fix for #24788778, to 5.7.
(The patch we actually ended up with in trunk relied on C++,
and ctype-utf8.c wasn't moved to C++ before 8.0.)
His original commit message follows:

SEVERE REGRESSION IN MY_STRNXFRM() FROM MYSQL-5.5 -> 5.6

Enhance the unicode-pad code to encourage the compiler
to loop-unroll. That improves the performance of
padding of utf8 char strings by 2x - 4x

This patch only takes care of utf8 space padding.
There seems to be similar oportunities for optimizing
other character encodings using the same technique as in this
patch.

Patch does not handle the regression caused by
my_strnxfrm_unicode() being called instead of
my_strnxfrm_mb_bin() as in mysql 5.5.

Change-Id: I17f6f2c7635851903ea7cf9b5f6e4146517c6e40