Key decleration of union semun on src version

FreeBSD is defined by the compiler derived from the triple. When
building FreeBSD 11 on a FreeBSD 12 with a CROSS_TOOLCHAIN=llvm10,
FreeBSD was set to 12 when building lib32 (for some reason no triple
is being passed which seems to mean that we're taking default values
from the build system). This in turn meant we end up with a double
decleration of union semun which is a build error.

Reviewed by: gshapiro, dim
Differential Revision: https://reviews.freebsd.org/D26902