Don't use an alternative stack for SIGSEGV & co.

Don't use an alternative stack for SIGSEGV & co.

This commit reverts most of c575766202773c858be0870c20cd495b722927c3, in
order to use back main stack for signal handling.

The main reason is that otherwise it is completely pointless that we do
a lot of efforts to print the stack trace on crash, and the content of
the stack and registers as well. Using an alternate stack broken this
feature completely.