Expand target bit set, shrink file size, and add loongarch64-linux-gnusf

[alexrp]

Expanding the target bit set from u32 to u64 results in an unacceptable size increase. So instead, I changed it to be written as a u64 in ULEB128. Even after adding loongarch64-linux-gnusf, the abilists file shrunk from 269 KB to 265 KB. I also moved the inclusion terminal bit over to the library index byte since that value is expected to be very small ~forever, shrinking the file further to 242 KB (because that bit was kind of undermining ULEB128). Finally, I changed the object size to be u16 written as ULEB128 too, resulting in a final size of 240 KB.

Closes #11.

[andrewrk]

Nice work, clever solution!