[Propeller] Use a bit-field struct for the metdata fields of BBEntry.

This patch encapsulates the encoding and decoding logic of basic block metadata into the Metadata struct, and also reduces the decoded size of `SHT_LLVM_BB_ADDR_MAP` section.

The patch would've looked more readable if we could use designated initializer, but that is a c++20 feature.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D148360