druntime.segfault-message.dd

New segfault handler showing backtraces for null access / call stack overflow on linux

While buffer overflows are usually caught by array bounds checks, there are still other situations where a segmentation fault occurs in D programs:

- `null` pointer dereference
- Corrupted or dangling pointer dereference in `@system` code
- Call stack overflow (infinite recursion)

These result in an uninformative runtime error such as:

$(CONSOLE
[1]    37856 segmentation fault (core dumped)  ./app
)

In order to find the cause of the error, the program needs to be run again in a debugger like GDB.

There is the `registerMemoryErrorHandler` function in `etc.linux.memoryerror`, which catches `SIGSEGV` signals and transforms them into a thrown `InvalidPointerError`, providing a better message.
However, it doesn't work on call stack overflow, because it uses stack memory itself, so the segfault handler segfaults.
It also relies on inline assembly, limiting it to the x86 architecture.

A new function `registerMemoryAssertHandler` has been introduced, which does handle stack overflow by setting up an [altstack](https://man7.org/linux/man-pages/man2/sigaltstack.2.html).
It uses `assert(0)` instead of throwing an `Error` object, so the result corresponds to the chosen `-checkaction` setting.

Example:

---
void main()
{
    version (linux)
    {
        import etc.linux.memoryerror;
        registerMemoryAssertHandler();
    }
    int* p = null;
    int* q = cast(int*) 0xDEADBEEF;

    // int a = *p; // segmentation fault: null pointer read/write operation
    // int b = *q; // segmentation fault: invalid pointer read/write operation
    recurse();     // segmentation fault: call stack overflow
}

void recurse()
{
    recurse();
}
---

Output with `dmd -g -run app.d`:

$(CONSOLE
core.exception.AssertError@src/etc/linux/memoryerror.d(82): segmentation fault: call stack overflow
$(NDASH)$(NDASH)$(NDASH)$(NDASH)$(NDASH)$(NDASH)$(NDASH)$(NDASH)$(NDASH)$(NDASH)
src/core/exception.d:587 onAssertErrorMsg [0x58e270d2802d]
src/core/exception.d:803 _d_assert_msg [0x58e270d1fb64]
src/etc/linux/memoryerror.d:82 _d_handleSignalAssert [0x58e270d1f48d]
??:? [0x7004139e876f]
./app.d:16 void scratch.recurse() [0x58e270d1d757]
./app.d:18 void scratch.recurse() [0x58e270d1d75c]
./app.d:18 void scratch.recurse() [0x58e270d1d75c]
./app.d:18 void scratch.recurse() [0x58e270d1d75c]
./app.d:18 void scratch.recurse() [0x58e270d1d75c]
...
...
...
)