[llvm-readelf/llvm-readobj] Remove one-dash long options

llvm-readelf is a user-facing tool which emulates GNU readelf. Remove one-dash
long options which are not recognized by GNU style `getopt_long`. This ensures
long options cannot collide with grouped short options.

Note: the documentation (D63719)/help messages have recommended the double-dash
forms since LLVM 9.0.0.
llvm-readobj is intended as an internal tool which has some flexibility.
llvm-readelf/llvm-readobj use the same option parsing code and llvm-readobj's
one-dash long options aren't used after test migration.

Differential Revision: https://reviews.llvm.org/D106037