gas: aarch64: Introduce the chericap directive

gas: aarch64: Introduce the chericap directive

This directive is the equivalent of the capinit directive except that it
allocates space as well as creating a CAPINIT relocation.
It is useful to be added to binutils since LLVM intend to transition
away from using capinit to chericap and this helps enable that
transition.

Here we just emit the required number of zeros into the output file with
md_number_to_chars.  Since we don't have to worry about endianness for a
big zero this is not complicated.