* Allow unused return value in cc source

With llvm-7.0.0,
some annoying warning messeges are raised:

	/home/lecopzer/workspace/bcc/src/cc/libbpf.c:456:3: warning: ignoring return value of ‘fgets’, declared with attribute warn_unused_result [-Wunused-result]
   	fgets(fmt, sizeof(fmt), f); // pos
   	^~~~~~~~~~~~~~~~~~~~~~~~~~

/home/lecopzer/workspace/bcc/src/cc/libbpf.c: In function ‘bpf_prog_get_tag’:
/home/lecopzer/workspace/bcc/src/cc/libbpf.c:456:3: warning: ignoring return value of ‘fgets’, declared with attribute warn_unused_result [-Wunused-result]
   fgets(fmt, sizeof(fmt), f); // pos
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
...

/home/lecopzer/workspace/bcc/tests/cc/utils.cc: In function ‘int cmd_scanf(const char*, const char*, ...)’:
/home/lecopzer/workspace/bcc/tests/cc/utils.cc:30:10: warning: ignoring return value of ‘int vfscanf(FILE*, const char*, __va_list_tag*)’, declared with attribute warn_unused_result [-Wunused-result]
   vfscanf(pipe, fmt, args);
   ~~~~~~~^~~~~~~~~~~~~~~~~

Let get rid of them by adding -Wno-unused-result.

* cc: Fix comparison between signed and unsigned value

With llvm-7.0.0:

/home/lecopzer/workspace/bcc/src/cc/common.cc: In function ‘std::__cxx11::string ebpf::get_pid_exe(pid_t)’:
/home/lecopzer/workspace/bcc/src/cc/common.cc:60:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (res >= sizeof(exe_path))
       ~~~~^~~~~~~~~~

As the declaration of `exe_path` is `char exe_path[4096]`,
the `sizeof(exe_path)` would always return 4096 (unsigned), so it's safe
to static cast to `int` unless it's larger than 2^31 - 1.