binutils-2_28-branch: RISC-V Backports

[Palmer Dabbelt]

Here's my latest round of RISC-V backports.  The first one is actually one I
missed, while the remainder are recent commits to master.  While I know there
might not be another release on the 2.28 branch, I'd like to keep backporting
our stuff as we've fixed a lot of bugs since 2.28 was released.

Are these OK for the 2.28 branch?

[PATCH 1/4] RISC-V/bfd: Hook elf_backend_object_p to set the mach
[PATCH 2/4] RISC-V: Resurrect GP-relative disassembly hints
[PATCH 3/4] RISC-V: Allow 32-bit BFD to handle 64-bit objects
[PATCH 4/4] RISC-V: Change CALL macro to use ra as the temporary