Add initial support for the floating point implementation register.

• Save the current FIR in the global 'cpuinfo' structure in a new 'fpu_id' member.
• Decode flags in the FIR when displaying other CPU flags during boot.
• Use the existing "dummy" slot in the floating point register structure to export the FIR in process core dumps and via ptrace(). Note that while the FIR register is not volatile, this practice of storing the FIR in the floating-point register set is used in other OS's.

Reviewed by: kan
MFC after: 1 month
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D10617