boot: Fix gdb support for RISC-V

This will also allow debugging systemd-boot easily on RISC-V.

Note that the following much simpler variant won't work, as we might be missing
the optional 'zihintpause' extension:

    asm volatile("pause");

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@codethink.co.uk>

src/boot/util.c

@@ -402,6 +402,8 @@ __attribute__((noinline)) void notify_debugger(const char *identity, volatile bo
                 asm volatile("pause");
 #  elif defined(__aarch64__)
                 asm volatile("wfi");
+#  elif defined(__riscv)
+                asm volatile(".insn i 0x0F, 0, x0, x0, 0x010");
 #  else
                 BS->Stall(5000);
 #  endif