[SIMD] Investigate memory use during compilation of vc tests

From !620 (comment 53538):

Huh. On sky, compilation can take nearly 8GB memory:

50495 joe       20   0 7597428 7.111g  21728 R 100.0  1.9   6:48.38 cc1plus                                                                                                                    
50496 joe       20   0 7619628 7.117g  21752 R 100.0  1.9   6:48.20 cc1plus                                                                                                                    

While compiling these four:

[ 85%] Building CXX object dune/common/simd/test/CMakeFiles/vctest.dir/vctest_vector_std_int16_t.cc.o
[ 85%] Building CXX object dune/common/simd/test/CMakeFiles/vctest.dir/vctest_mask_std_int16_t.cc.o
[ 85%] Building CXX object dune/common/simd/test/CMakeFiles/vctest.dir/vctest_vector_std_uint16_t.cc.o
[ 86%] Building CXX object dune/common/simd/test/CMakeFiles/vctest.dir/vctest_mask_std_uint16_t.cc.o

(Ubuntu 16.04, g++ 5.5.0, -O2 -g -Wall)

Might be specific to compiling on skylake-avx512, ubuntu 16.04, the g++-version or -O2 in combination with -g. At least I can compile on my laptop with -j4 (Ivy bridge (avx), Debian stretch, g++ 6.3, and just 8GB of ram).