correctly fix compilation of NEON_ASM sources

This reverts commit 1ef74a763a726829bfc26224d82acff207fdc8bb, as
assembler files in SOURCES break compiling with -pch, as we don't create
a respective PCH.

instead, compile assembler code with QMAKE_CC, not QMAKE_CXX.

the reason why this change is needed in the first place is not clear to
me, but i guess that CXX defines some c++-related macros when
preprocessing the file, which breaks further down the line. this is
counter-intuitive, as the g++ frontend should treat the same sources the
same way as the gcc frontend (differences should be limited the the ld
invocation).

Task-number: QTBUG-29765
Change-Id: Ic0116b3a5fa621f12ac41cadf3062ff00b538e85
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>