Named ABIs list should forward-reference ELF flags

[asb]

I've not submitted a PR for this as I don't have the asciidoc toolchain set up on this machine and I'm not sure if the doc is currently set up to allow cross-document cross-references.

Section 2.4 "Named ABIs" lists a whole bunch of ELF flags (e.g. EF_RISCV_FLOAT_ABI_SOFT) to clarify the ABI being discussed. This is helpful, but potentially confusing if you're not sure what is being referenced. This would be easily solved by changing "This specification defines the following named ABIs:" to "This specification defines the following named ABIs (relevant ELF flags are described in parentheses):