Specifying the target ISA for objdump to disassemble

[asb]

GNU objdump will currently assume the input binary could contain instructions from any standard extension. This could result in misleading results for binaries that target non-standard extensions that use some of the standard extension encoding space. Encoding the target ISA in an ELF section could allow ELF binaries to be "self describing" - see riscv-non-isa/riscv-elf-psabi-doc#47 for some discussion.