linux(4): Fix loadable modules after r367395

Move dtrace SDT definitions into linux_common module code. Also, build
linux_dummy.c into the linux_common kld -- we don't need separate
versions of these stubs for 32- and 64-bit emulation.

Reported by: several
PR: 250897
Discussed with: emaste, trasz
Tested by: John Kennedy, Yasuhiro KIMURA, Oleg Sidorkin
X-MFC-With: r367395
Differential Revision: https://reviews.freebsd.org/D27124