linux-user: Save the correct resume address for MIPS signal handling

linux-user: Save the correct resume address for MIPS signal handling

The current ISA mode needs to be saved in bit 0 of the resume address.
If the current instruction happens to be in a branch delay slot, then
the address of the preceding jump instruction should be stored instead.
exception_resume_pc already does both of these tasks, so it is
made available and reused.

MIPS_HFLAG_BMASK in hflags is cleared, otherwise QEMU may treat the
first instruction of the signal handler as a delay slot instruction.

Signed-off-by: Kwok Cheung Yeung <[email protected]>
Signed-off-by: Aurelien Jarno <[email protected]>