binfmt_flat cleanups and RISC-V support v2

[Christoph Hellwig]

Hi Greg,

below is a larger stash of cleanups for the binfmt_misc code,
preparing for the last patch that now trivially adds RISC-V
support, which will be used for the RISC-V nommu series I am
about to post.

Changes since v2:
 - fix the handling of old format flags
 - don't pass arguments on stack for RISC-V
 - small cleanups for flat_v2_reloc_t