Add auxv parsing to the architecture vector.

Necessary to fix parsing auxv entries from core files on systems that use
the layout specified by ELF instead of the incompatible variant used by Linux.

gdb/Changelog:

        * gdbarch.sh (auxv_parse): New.
        * gdbarch.h: Regenerated.
        * gdbarch.c: Regenerated.
        * auxv.c (target_auxv_parse): Call gdbarch_parse_auxv if provided.
Add gdbarch auxv parsing for OpenBSD.

gdb/Changelog:

        * obsd-tdep.c (obsd_auxv_parse): New function.
        (obsd_init_abi): Set auxv_parse.