trap: sigception: allow signal in signal handler context

this can hapen in combination with the static initializer:
(1) some code in class B wants to access a static field in class A
(2) SEGSEGV is issued.  we are in a signal context now
(3) this is the first usage of class A, so we have to load it
    and execute the static initializer
(4) in the static initializer of class A, there is also a static
    field access, which again causes a SIGSEGV

without `SA_NODEFER', the second SIGSEGV would be ignored.