imgact: Mark brandinfo and note structures as const

[zlei]

Summary

No functional change intended.

MFC after: 2 weeks
MFC with: 80336636b6b9f7a3bdad007c400e85eae017d2a2

[zlei]

This is for other architectures, i386, arm, powerpc and riscv.

Another brandnotes to be constified are freebsd_brandnote and kfreebsd_brandnote.

Discussed with Mark. Posted to review to see if I missed anything.

[kib]

sys/sys/imgact_elf.h

90

To add more to the const fest.

[jrtc27]

sys/sys/imgact_elf.h

90

None of the other fields are const though?

[kib]

sys/sys/imgact_elf.h

90

This is why I am talking about fest.

[zlei]

sys/sys/imgact_elf.h

90

const header_supported is now in final commit.