[RuntimeDyld] load all sections with ProcessAllSections

This patch tried to address the following use case.
  . bcc (https://github.com/iovisor/bcc

) utilizes llvm JIT to
    compile for BTF target.
  . with -g, .BTF and .BTF.ext sections (BPF debug info)
    will be generated by LLVM.
  . .BTF does not have relocations and .BTF.ext has some
    relocations.
  . With ProcessAllSections, .BTF.ext is loaded by JIT dynamic linker
    and is available to application. But .BTF is not loaded.

The bcc application needs both .BTF.ext and .BTF for debugging
purpose, and .BTF is not loaded. This patch addressed this issue
by iterating over all sections and loading any missing
sections, after symbol/relocation processing in loadObjectImpl().

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D55943

llvm-svn: 352432