arm: Implement kernel ifunc

Add kernel ifunc support on arm.

MFC after : 3 weeks
Reviewed by: kib (previous version)
Differential Revision: https://reviews.freebsd.org/D54970

arm, riscv: add a preprocessor symbol indicating missed support of ifunc

in kernel.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.free

arm, riscv: add a preprocessor symbol indicating missed support of ifunc

in kernel.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D54831

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