[PERF2BOLT/BOLT] Improve support for .so

Summary:
Avoid asserting on inputs that are shared libraries with
R_X86_64_64 static relocs and RELATIVE dynamic relocations matching
those. Our relocation checking mechanism would expect the result of
the static relocation to be encoded in the binary, but the linker
instead puts it as an addend in the RELATIVE dyn reloc.

Also fix aggregation for .so if the executable segment is not the
first one in the binary.

(cherry picked from FBD18651868)