QString, QJson, QHash: Fix UBs involving unaligned loads

Found by UBSan:

  src/corelib/tools/qstring.cpp:587:42: runtime error: load of misaligned address 0x2acbf4b7551b for type 'const long long int', which requires 8 byte alignment
  src/corelib/json/qjson_p.h:405:30: runtime error: store to misaligned address 0x0000019b1e52 for type 'quint64', which requires 8 byte alignment
  src/corelib/tools/qhash.cpp:116:27: runtime error: load of misaligned address 0x2b8f9ce80e85 for type 'const qlonglong', which requires 8 byte alignment
  src/corelib/tools/qhash.cpp:133:26: runtime error: load of misaligned address 0x2b8f9ce80e8d for type 'const ushort', which requires 2 byte alignment

Fix by memcpy()ing into a local variable. Wrap this trick in
template functions in qsimd_p.h. These are marked as always-
inline and use __builtin_memcpy() where available in an
attempt to avoid the memcpy() function call overhead in debug
builds.

While this looks prohibitively expensive, from the pov of the
C++ abstract machine, it is 100% equivalent, except for the
absence of undefined behavior. In one case, the cast produces
a local temporary which is then copied into the function, and
in the other case, that local variable comes from return value
of qUnalignedLoad().

Consequently, GCC compiles these two versions into identical
assembler code (only verfied for ucstrncmp, but there's no
reason to believe that it wouldn't hold for the other cases,
too).

Task-number: QTBUG-51651
Change-Id: Ia50d4a1d7580b6f803e0895c9f3d89c7da37840c
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>