scripts/libc: do not build add-ons by default

Currently, no --enable-add-ons option is passed to libc configure when
"$(do_libc_add_ons_list ,)" is empty, which makes configure automatically search
for present add-ons. In that case, all present add-ons are built, although
no add-on was selected by the user in the config. Moreover, this can make the
configure fail if some non-standard add-ons like eglibc-localedef are present.

This behavior also leads to an inconsistency from a user point of view between
the following cases:
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS="none" in the config,
which makes "$(do_libc_add_ons_list ,)" return "", so all present add-ons
are built.
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS!="none" in the
config, which makes "$(do_libc_add_ons_list ,)" return the add-on supporting
the chosen threading implementation, e.g. "nptl", so only this add-on is
built.

This patch disables the building of all add-ons in that case.

It is still possible to build all present add-ons by adding --enable-add-ons to
LIBC_GLIBC_EXTRA_CONFIG_ARRAY.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>

 diff -durN binutils-2.20.1.orig/bfd/elf64-ppc.c binutils-2.20.1/bfd/elf64-ppc.c

 --- binutils-2.20.1.orig/bfd/elf64-ppc.c	2010-02-03 14:28:25.000000000 +0100

 +++ binutils-2.20.1/bfd/elf64-ppc.c	2010-08-17 19:32:04.000000000 +0200

 @@ -11991,7 +11991,12 @@

  	      else if (!SYMBOL_REFERENCES_LOCAL (info, &h->elf)

  		       && !is_opd

  		       && r_type != R_PPC64_TOC)

 -		outrel.r_info = ELF64_R_INFO (h->elf.dynindx, r_type);

 +		{

 +		  outrel.r_info = ELF64_R_INFO (h->elf.dynindx, r_type);

 +		  if (h->elf.dynindx == -1

 +		      && h->elf.root.type == bfd_link_hash_undefweak)

 +		  memset (&outrel, 0, sizeof outrel);

 +		}

  	      else

  		{

  		  /* This symbol is local, or marked to become local,