Include TMPFS in all the GENERIC kernel configs

Include TMPFS in all the GENERIC kernel configs

Being able to use tmpfs without kernel modules is very useful when building
small MFS_ROOT kernels without a real file system.
Including TMPFS also matches arm/GENERIC and the MIPS std.MALTA configs.

Compiling TMPFS only adds 4 .c files so this should not make much of a
difference to NO_MODULES build times (as we do for our minimal RISC-V
images).

Reviewed By: br (earlier version for riscv), brooks, emaste
Differential Revision: https://reviews.freebsd.org/D25317