Add support of gcc __vector_size__

rtree-c/src/ReduceC.hs

rtree-c/test/cases/large/gcc-71626.c

rtree-c/test/expected/gcc-71626/extract.c

rtree-c/test/expected/gcc-71626/reduction/p0.path

rtree-c/test/expected/gcc-71626/reduction/p1.path

rtree-c/test/expected/gcc-71626/reduction/p2.path

rtree-c/test/expected/gcc-71626/reduction/p3.path

rtree-c/test/expected/gcc-71626/reduction/p4.path

rtree-c/test/expected/gcc-71626/reduction/p5.path

rtree-c/test/expected/gcc-71626/reduction/p6.path

rtree-c/test/expected/gcc-71626/reduction/p7.path

rtree-c/test/expected/gcc-71626/reduction/x0.c

rtree-c/test/expected/gcc-71626/reduction/x1.c

rtree-c/test/expected/gcc-71626/reduction/x2.c

rtree-c/test/expected/gcc-71626/reduction/x3.c

rtree-c/test/expected/gcc-71626/reduction/x4.c

rtree-c/test/expected/gcc-71626/reduction/x5.c

rtree-c/test/expected/gcc-71626/reduction/x6.c

rtree-c/test/expected/gcc-71626/reduction/x7.c

rtree-c/test/src/ReduceCSpec.hs