MIPS: mark O32+FP64 experimental for now

MIPS: mark O32+FP64 experimental for now

Commit 597ce1723e0f "MIPS: Support for 64-bit FP with O32 binaries"
introduced support for setting Status.FR=1 for O32 binaries with the
EF_MIPS_FP64 ELF header flag set. Whilst this flag is currently
supported by binutils it does introduce an ABI break within userland.
Objects built with EF_MIPS_FP64 cannot be safely linked with those built
without it since code in either object may assume behaviour specific to
a value of FR.

More recently there has been discussion around avoiding further
fragmentation of the O32 ABI whilst still allowing the use of FR=1 and
features such as MSA which depend upon it. Details of the plan to allow
this are still being worked on, and whilst the kernel will need the
ability to handle FR=1 with O32 tasks it is unclear what else it may
need to provide to a userland which seeks to avoid another ABI break. In
order to prevent the proliferation of userland which may rely upon the
current EF_MIPS_FP64 behaviour this patch marks the kernel support for
it experimental & disables it by default. Under current proposals it is
likely that this support can simply be enabled again later, but possibly
after the introduction of further interfaces with userland and support
for the MIPS R5 UFR feature.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Paul Burton <paul.burton@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/6549/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>