gdb/testsuite: Declare that riscv*-*-linux* cannot hardware_single_step

gdb/testsuite: Declare that riscv*-*-linux* cannot hardware_single_step

Many tests fail in gdb/testsuite/gdb.base/sigstep.exp on
riscv64-linux-gnu.  Those tests check that when stepping, if the
debuggee received a signal it should step inside the signal handler.

This feature requires hardware support for single stepping (or at least
kernel support), but none are available on riscv*-linux-gnu hosts, at
the moment at least.

This patch adds RISC-V to the list of configurations that does not
have hardware single step capability, disabling tests relying on such
feature.

Tested on riscv64-linux-gnu.