Description:kernelspace musl code

Team:OTHERS
Feature or Bugfix:Feature
Binary Source:NA
PrivateCode(Yes/No):No

Change-Id: I99874a7c570b7d22b4a3d34840260eb48ea3ffa1
Reviewed-on: http://mgit-tm.rnd.huawei.com/10276995Reviewed-by: Nshenwei 00579521 <denny.shenwei@huawei.com>
Tested-by: Npublic jenkins <public_jenkins@notesmail.huawei.com>

despite clarifications made to the COPYRIGHT file in commit
f0a61399, there continues to be
confusion about whether the permissions granted actually apply to all
files. I am the sole author of these files and clearly intend, and
have always intended, for the grant of permission to apply to them.

this patch makes the functions which work directly on multibyte
characters treat the high bytes as individual abstract code units
rather than as multibyte sequences when MB_CUR_MAX is 1. since
MB_CUR_MAX is presently defined as a constant 4, all of the new code
added is dead code, and optimizing compilers' code generation should
not be affected at all. a future commit will activate the new code.

as abstract code units, bytes 0x80 to 0xff are represented by wchar_t
values 0xdf80 to 0xdfff, at the end of the surrogates range. this
ensures that they will never be misinterpreted as Unicode characters,
and that all wctype functions return false for these "characters"
without needing locale-specific logic. a high range outside of Unicode
such as 0x7fffff80 to 0x7fffffff was also considered, but since C11's
char16_t also needs to be able to represent conversions of these
bytes, the surrogate range was the natural choice.

these functions were setting wc to point to wchar_t aliasing itself as
a "cheap" way to support null wc arguments. doing so was anything but
cheap, since even without the aliasing violation, it would limit the
compiler's ability to optimize.

making wc point to a dummy object is equally easy and does not suffer
from the above problems.

the interface contract for mbtowc admits a much faster implementation
than mbrtowc can achieve; wrapping mbrtowc with an extra call frame
only made the situation worse.

since the regex implementation uses mbtowc already, this change should
improve regex performance too. it may be possible to improve
performance in other places internally by switching from mbrtowc to
mbtowc.

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.