ARM: vfp: Reimplement VFP exception entry in C code

En/disabling softirqs from asm code turned out to be trickier than
expected, so vfp_support_entry now returns by tail calling
__local_enable_bh_ip() and passing the same arguments that a C call to
local_bh_enable() would pass. However, this is slightly hacky, as we
don't want to carry our own implementation of local_bh_enable().

So let's bite the bullet, and get rid of the asm logic in
vfp_support_entry that reasons about whether or not to save and/or
reload the VFP state, and about whether or not an FP exception is
pending, and only keep the VFP loading logic as a function that is
callable from C.

Replicate the removed logic in vfp_entry(), and use the exact same
reasoning as in the asm code. To emphasize the correspondence, retain
some of the asm comments in the C version as well.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>