ELF: Create synthetic sections for loadable partitions.

We create several types of synthetic sections for loadable partitions, including:

• The dynamic symbol table. This allows code outside of the loadable partitions to find entry points with dlsym.
• Creating a dynamic symbol table also requires the creation of several other synthetic sections for the partition, such as the dynamic table and hash table sections.
• The partition's ELF header is represented as a synthetic section in the combined output file, and will be used by llvm-objcopy to extract partitions.

Differential Revision: https://reviews.llvm.org/D62350