RELOC_AGAINST_DISCARDED_SECTION zero size reloc sections

RELOC_AGAINST_DISCARDED_SECTION zero size reloc sections

For some reason the initial implementation (commit 0672748ac053) of
this macro didn't allow discarding of all relocs in a section, perhaps
because doing so would require a testsuite change.  This patch allows
zero size relocation sections to result, and adjusts the testsuite.
i386, x86_64, ppc and ppc64 code that avoids a memmove is also changed
to allow zero size reloc sections, and arc fixed to actually adjust
the reloc section header.