platforms: drop 12.x, remove unsupported platforms

With the EOL of FreeBSD 12.x, no supported branch supports 32-bit
ARMv4/v5 (arm) or 64-bit SPARCv9 (sparc64).

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42680