Blackfin: fix elf_fpregset_t definition

The elf_fpregset_t type relied on an empty struct in the asm/user.h, but
the transition to asm-generic/user.h dropped that empty struct.  Rather
than restore this useless struct, define the only user (elf_fpregset_t)
as an empty struct itself.  This fixes building when ELF dump support is
enabled.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>