Disable 64-bit atomics on 32-bit x86

At least with GCC, the use of cmpxchg8b in inline assembly is
unreliable. The instruction requires 5 registers to be used and
sometimes GCC complains that it runs out of them.

qatomic_x86.h:424:33: error: can’t find a register in class ‘GENERAL_REGS’ while reloading ‘asm’
qatomic_x86.h:424:33: error: ‘asm’ operand has impossible constraints

Change-Id: Ie5414f3bccc6e559c7eec93beabe8663ab40271f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Doc: don't promise a null QString if we can't be held to it

QDirIterator::next() returns QDirIterator::filePath(), which returns
QFileInfo::filePath() on a default-constructed QFileInfo. That
specifically returns a non-null QString for some reason. I don't know
why but I won't change it.

Task-number: QTBUG-35422
Change-Id: I6bbcf8b83153e44c36d6320d27ce223ef28503ba
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>