RISC-V: Don't increment sepc after breakpoint.

Adding 4 to sepc is pointless, and is wrong if we executed a 2-byte
compressed breakpoint.  This plus a corresponding gdb patch allows
compressed breakpoints to work in gdb.  Gdb maintainers have already
agreed that this is the right approach.
Signed-off-by: NJim Wilson <jimw@sifive.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/traps.c

@@ -138,7 +138,6 @@ asmlinkage void do_trap_break(struct pt_regs *regs)
 #endif /* CONFIG_GENERIC_BUG */
 
 	force_sig_fault(SIGTRAP, TRAP_BRKPT, (void __user *)(regs->sepc), current);
-	regs->sepc += 0x4;
 }
 
 #ifdef CONFIG_GENERIC_BUG