[elfabi] Add support for reading DT_SONAME from binaries
This change gives the llvm-elfabi tool the ability to read DT_SONAME from a binary ELF file into an ELFStub.
- DynamicEntries struct for storing dynamic entries that are relevant to elfabi.
- terminatedSubstr() retrieves a null-terminated substring from a StringRef.
- appendToError() appends a string to an error, allowing more specific error messages.
Differential Revision: https://reviews.llvm.org/D55629