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>

 Original patch from Gentoo.

 -= BEGIN original header =-

 ARMV5 can use STRD and LDRD access instructions but these accesses need to be 

 8 byte aligned. The dynamic linker's malloc needs to match this so structures

 become 8 byte aligned to void unaligned accesses.

 RP - 14/02/2008

 -= END original header =-

 diff -durN uClibc-0.9.30.orig/ldso/ldso/arm/dl-sysdep.h uClibc-0.9.30/ldso/ldso/arm/dl-sysdep.h

 --- uClibc-0.9.30.orig/ldso/ldso/arm/dl-sysdep.h	2008-09-25 10:35:20.000000000 +0200

 +++ uClibc-0.9.30/ldso/ldso/arm/dl-sysdep.h	2009-02-07 09:57:59.000000000 +0100

 @@ -15,6 +15,8 @@

    GOT_BASE[1] = (unsigned long) MODULE; \

  }

 +#define DL_MALLOC_ALIGN 8  /* EABI needs 8 byte alignment for STRD LDRD*/

 +

  static __inline__ unsigned long arm_modulus(unsigned long m, unsigned long p)

  {

  	unsigned long i,t,inc;