libtool: avoid non-underscore symbols in the "dumpbin -symbols" wrapper

libtool: avoid non-underscore symbols in the "dumpbin -symbols" wrapper

If an object has a symbol lacking an expected leading underscore,
the symbol name is not printed, but the symbol type is, leading
to output such as:
    T _normal _normal
    T T _another _another
when the expected output would have been something like:
    T _normal _normal
    T no_underscore no_underscore
    T _another _another
However, symbols lacking an expected leading underscore are
not "real" symbols, they are internal symbols which we don't
care about, therefore drop them instead.
* m4/libtool.m4 (_LT_CMD_GLOBAL_SYMBOLS) [MSVC]: Output the whole
symbol line in one go.
* NEWS: Update.