[Ada] sigaction result not properly checked in __gnat_install_handler (QNX)

[Ada] sigaction result not properly checked in __gnat_install_handler (QNX)

The QNX version of __gnat_install_handler calls sigaction for a number
of signals, and then prints an error message when the the call failed.
But unfortunately, except for the first call, we forgot to store
sigaction's return value, so the check that ensues uses a potentially
uninitialized variable, which the compiler does detect (this is how we
found this issue).

This change fixes this by make sure we store the result of each
sigaction call before checking it.

While at it, we noticed a thinko in the error messages all alerting
about the SIGFPE signal, rather than the signal it just tested.  Most
likely a copy/paste thinko. Fixed by this change as well.

gcc/ada/

	* init.c (__gnat_install_handler) [__QNX__]: Save sigaction's
	return value in err before checking err's value.  Fix incorrect
	signal names in perror messages.