core: fix x86 PIC code compilation

This bug was triggered by Buildroot autobuilders [1,2], causing this
kind of failures [3,4]:

  [ 14%] Building CXX object modules/core/CMakeFiles/opencv_core.dir/src/system.cpp.o
  /home/test/autobuild/instance-0/output/build/opencv-2.4.10/modules/core/src/system.cpp: In function '(static initializers for /home/test/autobuild/instance-0/output/build/opencv-2.4.10/modules/core/src/system.cpp)':
  /home/test/autobuild/instance-0/output/build/opencv-2.4.10/modules/core/src/system.cpp:280:10: error: inconsistent operand constraints in an 'asm'
  make[3]: *** [modules/core/CMakeFiles/opencv_core.dir/src/system.cpp.o] Error 1

[1] http://buildroot.org/
[2] http://autobuild.buildroot.org/
[3] http://autobuild.buildroot.org/?reason=opencv-2.4.10
[4] http://autobuild.buildroot.org/results/483/4838285b25d6293a5cf0bb9eadd5040a7c75d766/build-end.log

Signed-off-by: Samuel Martin <s.martin49@gmail.com>