aix: make ctype_inline.h thread-safe and avoid _OBJ_DATA char subscript.

aix: make ctype_inline.h thread-safe and avoid _OBJ_DATA char subscript.

g++.dg/warn/Wstringop-overflow-6.C tests for a bogus overflow warning in
system headers.  This testcase was generating a -Wchar-subscript warning
on AIX because ctype_inline.h was subscripting AIX _OBJ_DATA using a char.
The _M_table case cast the subscript to unsigned char, but the _OBJ_DATA
case did not.

The investigation also exposed that AIX has added a thread-safe variant
of access to __lc_type that had not been applied to the libstdc++
implementation.

This patch casts the subscript to unsigned char and adds the THREAD_SAFE
variant.  libstdc++ always is compiled with pthreads, but it is good
to make the situation explicit and to document the appropriate usage.

Bootstrapped on powerpc-ibm-aix7.2.3.0.

libstdc++-v3/ChangeLog:

	* config/os/aix/ctype_inline.h (bool ctype<char>:: is): Cast
	_OBJ_DATA subscript to unsigned char. Add _THREAD_SAFE access to
	__lc_type.
	(const char* ctype<char>:: is): Same.