[llvm-nm] Add --format=just-symbols and make --just-symbol-name its alias

https://sourceware.org/bugzilla/show_bug.cgi?id=27487 binutils will have
--format=just-symbols/-j as well.

Arbitrarily prefer `-j` to `--format=sysv`. Previously `--format=sysv -j` prints
in the sysv format while `-j` takes precedence over other formats.

Differential Revision: https://reviews.llvm.org/D98569