Add support for breaking into debugger for Linux

Use Linux-specific /proc/$PID/status file to check whether we're being
debugged and a generic raise(SIGTRAP) to actually break into the debugger.

include/internal/catch_debugger.h

@@ -35,6 +35,10 @@ namespace Catch{
         #endif
     #endif
 
+#elif defined(CATCH_PLATFORM_LINUX)
+    #include <signal.h>
+
+    #define CATCH_BREAK_INTO_DEBUGGER() if( Catch::isDebuggerActive() ) { raise(SIGTRAP); }
 #elif defined(_MSC_VER)
     #define CATCH_BREAK_INTO_DEBUGGER() if( Catch::isDebuggerActive() ) { __debugbreak(); }
 #elif defined(__MINGW32__)