[ia64] teach elf_access_reg() to handle the missing range (r16..r31)

Now it's easy to make elf_access_gpreg() handle the rest of global
registers (r16..r31).  That gets rid of the hole in the registers
elf_access_reg() can handle, which will allow to simplify its callers
later in the series.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>