target-xtensa: avoid double-stopping at breakpoints

env->exception_taken is set every time an exception is taken. It is used
to allow single-stepping to stop at the first exception handler
instruction. This however must exclude debug exceptions, as otherwise
first step from the instruction where breakpoint was hit stops at that
same instruction.
Also don't check env->exception_taken directly from the
gen_intermediate_code_internal, instead allocate and use TB flag
XTENSA_TBFLAG_EXCEPTION.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
tests/tcg/xtensa: Fix out-of-tree build

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: don't generate dead code to access invalid SRs

This fixes the following test failure caused by access to undefined SR:

    qemu-system-xtensa -M sim -cpu dc232b -nographic -semihosting  -kernel ./test_sr.tst
    QEMU 1.4.50 monitor - type 'help' for more information
    (qemu) QEMU 1.4.50 monitor - type 'help' for more information
    (qemu) qemu-system-xtensa: tcg/tcg.c:1673: temp_save: Assertion `s->temps[temp].val_type == 2 || s->temps[temp].fixed_reg' failed.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: check register window inline

This lowers time spent in helper_window_check as reported by perf top
from ~8% to ~0.15% accelerating register-intensive tests by ~20%.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>