Improve MACHINE_ARCH handling for hard vs soft-float on RISC-V.

For userland, MACHINE_ARCH reflects the current ABI via preprocessor
directives. For the kernel, the hw.machine_arch sysctl uses the ELF
header flags of the current process to select the correct MACHINE_ARCH
value.

Reviewed by: imp, kp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24543