Use clang 7 as compiler on 11.x and 12.x to fix build.

clang 8 and later error for duplicate definitions of enum values. mdb
depends on having identical but duplicate definitions. This was fixed
differently in 13 with a change to the kernel, but that change is
disruptive and cannot be easily backported.

Reported by: pkg-fallout, Attila Nagy
Approved by: bdrewery
MFH: 2019Q3
Differential Revision: https://reviews.freebsd.org/D22486