add GC benchmark suite

This benchmark suite was taken from http://git.llucax.com.ar/w/software/dgc/dgcbench.git with small modifications to build on current dmd. @leandro-lucarella-sociomantic Is this ok for you? Did you need permission of the original authors?

runbench.d is replaced with a version derived from d_do_test.d in the hope that some of its features will be useful in the future (specifying compile/test parameters in the source file, building with other compilers, keeping output in a results directory, etc.).
It greps output for "maxGCMemory" as provided by https://github.com/D-Programming-Language/druntime/pull/817 to add information to the console. This is an example:

```
OS=win64
GC_OPT=profile=1 reserve=0 minPoolSize=1 maxPoolSize=32 incPoolSize=1
 . bh.d             10.528 s, maxGCMemory = 28 MB, pause time = 1795 ms
 . bigarr.d          4.653 s, maxGCMemory = 28 MB, pause time = 3250 ms
 . bisort.d          0.978 s, maxGCMemory = 36 MB, pause time = 99 ms
 . conalloc.d        2.193 s, maxGCMemory = 6 MB, pause time = 260 ms
 . concpu.d          3.562 s, maxGCMemory = 6 MB, pause time = 1909 ms
 . em3d.d            3.682 s, maxGCMemory = 171 MB, pause time = 1940 ms
 . huge_single.d     0.022 s, maxGCMemory = 1 MB, pause time = 1 ms
 . mcore.d           0.105 s, maxGCMemory = 57 MB, pause time = 2 ms
 . rand_large.d      2.481 s, maxGCMemory = 78 MB, pause time = 1184 ms
 . rand_small.d      1.248 s, maxGCMemory = 10 MB, pause time = 698 ms
 . sbtree.d          0.021 s, maxGCMemory = 1 MB, pause time = 1 ms
 . split.d           0.868 s, maxGCMemory = 793 MB, pause time = 2 ms
 . testgc3.d        30.812 s, maxGCMemory = 688 MB, pause time = 19482 ms
 . tree1.d          29.696 s, maxGCMemory = 153 MB, pause time = 16769 ms
 . tree2.d           0.261 s, maxGCMemory = 10 MB, pause time = 43 ms
 . tsp.d             1.310 s, maxGCMemory = 66 MB, pause time = 270 ms
 . voronoi.d         4.903 s, maxGCMemory = 153 MB, pause time = 3793 ms
 . bulk.d           138.806 s, maxGCMemory = 1336 MB, pause time = 73594 ms
 . resize.d          1.796 s, maxGCMemory = 6 MB, pause time = 1 ms
 . string.d          0.064 s, maxGCMemory = 10 MB, pause time = 5 ms
```