Since in b0491307, which has introduced the optimized window spill
procedure, _xt_context_save did not work correctly when called from
_xt_syscall_exc. This was because unlike _xt_lowint1, _xt_syscall_exc
does not save PS and EPC1. The new version of _xt_context_save
modified PS (on purpose) and EPC1 (accidentally, due to window
overflow exceptions), which resulted in a crash upon 'rfi' from the
syscall.
This commit adds restoring of PS and EPC1 in _xt_context_save. It also
slightly reduces the number of instructions used to prepare PS for
window spill.
Unit test for setjmp/longjmp (which were broken by this regression)
is added.
Closes https://github.com/espressif/esp-idf/issues/4541
