Fix GPI-2 and enable TAGASPI

The rdma-core driver.h include is no longer installed:

 56dd87acd2

So ibv_read_sysfs_file() is not defined. As the symbols is still
distributed, we simply add the missing prototype manually.

Similarly, the gaspi_size_t gaspi_get_system_mem() function is not
available from the gaspi public headers, so we define it in the
max_mem.c test.

pkgs/gpi-2/default.nix

@@ -34,6 +34,8 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  patches = [ ./rdma-core.patch ./max-mem.patch ];
+
   preConfigure = ''
     patchShebangs autogen.sh
     ./autogen.sh