[Toybox] [PATCH] Fix wc -m on bionic.

[enh]

When mbrtowc returns -2, all n bytes have been processed. Bionic's
interpretation of POSIX is that you must not re-supply those bytes
on the next call, and should only supply the bytes needed to complete
the character. If you re-supply the bytes on the next call, bionic
considers that an illegal sequence and returns -1.

With these changes, the tests still pass on glibc and also pass on bionic.
---
 tests/wc.test   |  7 +------
 toys/posix/wc.c | 34 ++++++++++++++--------------------
 2 files changed, 15 insertions(+), 26 deletions(-)
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