Extending support for non-allocatable sections.

Summary:
The is a set of changes that allow modification of non-allocatable
sections in ELF binary. Primarily for the purpose of updating debug
info.

Extend LLVM interface to allow processing relocations in non-allocatable
sections. This allows to produce .debug* sections with resolved
relocations against generated code.

Extend BOLT rewriting framework to allow appending contents to
non-allocatable sections in the binary.

Re-worked ELF binary rewriting to support the above and to allow future
extensions (e.g. new section names).

(cherry picked from FBD3023403)