linux: provide just one instance of futex_list

Move futex_list definition to linux.c which is included once
in linux.ko (i386) and in linux_common.ko (amd64 and aarch64)
allowing 32/64 bit linux programs to access the same futexes
in the latter case.

PR: 240989
Reviewed by: dchagin
Differential Revision: https://reviews.freebsd.org/D22073