arm64: Enable the floating-point exception traps

To enable it user-space needs to call feenableexcept().

FPE_FLTIDO has been added as the IDF bit can't be mapped to any existing
FPE code.

Reviewed by: andrew@
Differential revision: https://reviews.freebsd.org/D35247
MFC after: 2 weeks

(cherry picked from commit 6e2caba7a1297eeda4f43e6377667d2e049f24f9)