The main purpose of this API is to proactively release llvm/clang
.text memory which is brought in during compilation.
bcc .text memory for some other functions, e.g., attach_tracepoint,
bpf_prog_load, etc. can also be freed after all these tasks are done.

Note that such memory is reclaimable in kernel since it has
file backup. But certain applicaiton may want to reduce this
memory immediately to satisfy constraints imposed by sysadmin, etc.

The implementation uses madvise with MADV_DONTNEED.
For the case where bcc is static linked into the binary,
we do not really know the start and the end of memory regions
used by bcc, so the implementation here bluntly returned
all .text memory back to kernel. This will incur some performance
overhead as later on executed instructions will need to bring
back to memory again.

For static linked library, instrumented RandomRead example,
without this patch, the RSS memory before load is:
  VmRSS:     63644 kB
  RssAnon:           23876 kB
  RssFile:           39768 kB
  RssShmem:              0 kB

After this patch,
  VmRSS:     34264 kB
  RssAnon:           23880 kB
  RssFile:           10384 kB
  RssShmem:              0 kB

For shared library, a python unit test, test_free_llvm_memory.py, is
added, which shows for a do-nothing bpf program, we have
  Before freeing llvm memory: RssFile:  43000 kB
  After  freeing llvm memory: RssFile:  11992 kB

The RssFile reduction on Facebook internal applications
also ranges in 30-40MB.
Signed-off-by: Yonghong Song <yhs@fb.com>