Fix mini_debug_info stripping when no symbols should be kept.

If there are no symbols to keep (i.e. all function symbols are also in
the dynsym table), then the $@.keep_symbols file has size 0.
When objcopy parses a --keep-symbols file, it has a special case for
0-sized files where it silently fails (exits with status 1, no error
message). On the other hand, objcopy is happy with a file containing no
symbols, as long as there is some whitespace or a comment. Avoid the
special case by appending a newline to keep_symbols.

Bug: b/62815515
Bug: b/77242617
Test: manual
Change-Id: I348593164fe78ece5c85db3d28c7daccce3a52fb