[llvm-readobj] Only print the real size of the note

Note payloads are padded to a multiple of 4 bytes in size, but the size
of the string that should be print can be smaller e.g. the n_descsz
field in gold's version note is 9, so that's the whole size of the
string that should be printed. The padding is part of the format of a
SHT_NOTE section or PT_NOTE segment, but it's not part of the note
itself.

Printing the extra null bytes may confuse some tools, e.g. when the
llvm-readobj is sent to grep, it treats the output as binary because
it contains a null byte.

Differential Revision: https://reviews.llvm.org/D30804

llvm-svn: 299576