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: Jim Wilson <jimw@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>