target/microblaze: Convert brk and brki to decodetree

target/microblaze: Convert brk and brki to decodetree

Split these out of the normal branch instructions, as they require
special handling.  Perform the entire operation inline, instead of
raising EXCP_BREAK to do the work in mb_cpu_do_interrupt.

This fixes a bug in that brki rd, imm, for imm != 0x18 is not
supposed to set MSR_BIP.  This fixes a bug in that imm == 0 is
the reset vector and 0x18 is the debug vector, and neither should
raise a tcg exception in system mode.

Introduce EXCP_SYSCALL for microblaze-linux-user.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>