[VIRT-327 # Richard's upstream QEMU work ]

More time than I expected on float128_muladd, adjusting the codebase to share code with float64_muladd.

[VIRT-339 # ARMv8.5-BTI, Branch Target Identification ]

Posted v12, adjusting one of the smoke tests vs a distro linker bug (fixed in mainline).

r~