gcc: enable multilib for target gcc

gcc: enable multilib for target gcc

- add a task to setup multilib configuration for target gcc
- this commit adapts Nitin Kamble's work to gcc 4.7
- use a hash for storing arch-dependent multilib options
- patch gcc in order to use the multilib config files from the
build directory

Tests:
root@qemux86-64:~# gcc -m64 t.c -o t
root@qemux86-64:~# file t
t: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.16, not stripped
root@qemux86-64:~# ./t
Hello World !
root@qemux86-64:~# gcc -m32 t.c -o t
root@qemux86-64:~# file t
t: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.16, not stripped
root@qemux86-64:~# ./t
Hello World !

[YOCTO #1369]

(From OE-Core rev: b26819c85881e82ee1b5c68840011e78c321f18e)

Signed-off-by: Constantin Musca <constantinx.musca@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>