GAS: Unify code for SET_SECTION_RELOCS call

GAS: Unify code for SET_SECTION_RELOCS call

Fold a separate call to `bfd_set_reloc' into SET_SECTION_RELOCS itself,
so that the GAS interface to this facility is contained in a single
invocation.

Currently both `write_relocs' and `obj_mach_o_reorder_section_relocs'
call `bfd_set_reloc', causing the function to be called twice by Mach-O
targets, such as `i386-darwin', once before target-specific processing
and again afterwards, which is at the very least fragile in terms of
assuming that any actions made by the function on the first invocation
won't interfere with the final intended result.

Set the macro by default to a plain call to `bfd_set_reloc', letting
backends override the macro, with the requirement now to factor in a
call to said function at the appropriate time.  Backends can choose
whether to call `bfd_set_reloc' first (such as COFF), or last (such as
Mach-O), or at any other point in relation to their own additional
actions.

Update the COFF variant accordingly, moving it to a new function for a
better code structure, retaining functionality.

This is in preparation for `bfd_set_reloc' to return an error status.