[debugserver] Add option to propagate SIGSEGV to target process

Adds a command line option that makes debugserver propagate the SIGSEGV
signal to the target process.

Motivation: I'm one of the maintainers of Delve [1] a debugger for Go.
We use debugserver as our backend on macOS and one of the most often
reported bugs is that, on macOS, we don't propagate SIGSEGV back to the
target process [2]. Sometimes some programs will actually cause a
SIGSEGV, by design, and then handle it. Those programs can not be
debugged at all.

Since catching signals isn't very important for a Go debugger I'd much
rather have a command line option in debugserver that causes it to let
SIGSEGV go directly to the target process.

[1] https://github.com/go-delve/delve/
[2] https://github.com/go-delve/delve/issues/852

Differential revision: https://reviews.llvm.org/D89315