Print a stack trace on unexpected term signal (e.g. SIGABRT)

For OSX and Linux, add a signal handler to SIGABRT, SGIFPE, SIGILL,
SIGSEGV, and SIGBUS. When the program terminates unexpectedly the
handler will print the current stack to the terminal to help determine
the location of the failure.

On OSX the output will look like:

```
Stack trace:
4   zstd                                0x000000010927ed96 main + 16886
5   libdyld.dylib                       0x00007fff767d1015 start + 1
6   ???                                 0x0000000000000001 0x0 + 1
```

On Linux the output will look like:

```
Stack trace:
./zstd() [0x4b8e1b]
./zstd() [0x4b928a]
./zstd() [0x403dc2]
/lib64/libc.so.6(__libc_start_main+0xf5) [0x7f5e0fbb0445]
./zstd() [0x405754]
```

As is, the code does not function on WIN32.

See also: https://oroboro.com/stack-trace-on-crash/

programs/fileio.c

@@ -157,7 +157,7 @@ static void clearHandler(void)
 }
 
 
-/* ************************************************************
+/*-************************************************************
 * Avoid fseek()'s 2GiB barrier with MSVC, macOS, *BSD, MinGW
 ***************************************************************/
 #if defined(_MSC_VER) && _MSC_VER >= 1400