[Ada] multilib patch take two => multilib build working

[Paolo Bonzini]

> Paolo, do you know where to look for the list of multilibs on a given platform
> in the GCC sources? And if we want to disable some of them for Ada?

In the makefile fragments t-*, in places like config/i386/t-linux64

MULTILIB_OPTIONS = m64/m32
MULTILIB_DIRNAMES = 64 32
MULTILIB_OSDIRNAMES = ../lib64 $(if $(wildcard $(shell echo 
$(SYSTEM_HEADER_DIR))/../../usr/lib32),../lib32,../lib)

Problematic (from an Ada point of view) configurations are those like 
config/arm/t-xscale-elf:

MULTILIB_OPTIONS     = mbig-endian
MULTILIB_DIRNAMES    = be
MULTILIB_EXCEPTIONS  =
MULTILIB_MATCHES     = mbig-endian=mbe mlittle-endian=mle

and 32/64-bit configurations like x86_64-pc-linux-gnu.

Paolo