Makefile.in: Allow to pass additional test flags

This allows to specify the test cases to be run.
For example the following command can be used to restrict the testcases
that will be executed to zb*.c and sm*.c from the directory
`gcc/testsuite/gcc.target/riscv/` (which includes the file `riscv.exp`):

  RUNTESTFLAGS="riscv.exp=zb*.c\ sm*.c" make report

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>