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