AArch64/ARM64: accept and print floating-point immediate 0 as "#0.0"

It's been decided that in the future, the floating-point immediate in
instructions like "fcmeq v0.2s, v1.2s, #0.0" will be canonically "0.0", which
has been implemented on AArch64 already but not ARM64.

This fixes that issue.

llvm-svn: 207666