Add support for single-file multi-target intrinsics in Qt

GCC 4.9 now allows us to #include any and all intrinsics headers, not
just the one for which we're compiling code, a behavior that ICC and
MSVC have had for some time. With that, we're able to have the functions
for different targets in the same source file. See the GCC manual:
  http://gcc.gnu.org/onlinedocs/gcc/Function-Multiversioning.html

This functionality is notified by the QT_COMPILER_SUPPORTS_HERE(XXX)
macro, which indicates that all the intrinsics from
QT_COMPILER_SUPPORTS_xxx are available and enabled. To complement, a
QT_COMPILER_SUPPORTS(XXX) macro is also added.

Unlike ICC and MSVC, GCC requires a special function attribute, which
will also cause code optimization. That's the QT_FUNCTION_TARGET macro.

Note: because of the absence of the target attribute, ICC and MSVC will
not generate instructions with the VEX prefix unless they only exist
with the VEX prefix or if -mavx / -arch:AVX are enabled.

Change-Id: I0c1880c20324bd8e0fc68a863e36d1fa7755dff0
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>

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