[DWARFYAML] Make the opcode_base and the standard_opcode_lengths fields optional.

This patch makes the opcode_base and the standard_opcode_lengths fields
of the line table optional. When both of them are not specified,
yaml2obj emits them according to the line table's version.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D88355