clang: default to DWARF 4 for FreeBSD 12.0 and later

Older FreeBSD versions included GDB 6.1 and had other tools that were
unable to handle debug information newer than DWARF 2.  Those tools have
since been updated.  (An old version of GDB is still kept for kernel
crash handling, but the kernel is compiled with an explicit -gdwarf2.)

Reviewed by:	dim
Differential Revision:	https://reviews.llvm.org/D66760

llvm-svn: 370779