Enable MONO_ARCH_EMULATE_FCONV_TO_U4 on PPC (mono/mono#15991)
Fixes regression related to floating point introduced in mono/mono#15930.
This does mean we have to take a slower path instead of using the
PPC instruction, but it provides consistent results. Other CPUs
have to do this too, so it's not like PPC is impacted unfairly.
(i.e, NaN gives an inconsistent result, at least with POWER7 and
some amd64 CPUs tested)
Fixes mono/mono#15990.
Commit migrated from https://github.com/mono/mono/commit/
e70ac8c6ae7ff3f03517be90babb8f7c25d18b97