Most of the tests in gcc.c-torture/execute/builtins that use LTO torture
options "-O2 -flto -fuse-linker-plugin -fno-fat-lto-objects" fail to
link on EABI and ELF targets with multiple definitions for either memset
or strlen.  I filed PR testsuite/55994 and was told that the fix is to
run the tests for those targets with "-Wl,--allow-multiple-definition".
This patch adds that option to the flags used for the builtins tests
for EABI and ELF targets.

That option is probably also needed for RTEMS targets; I've copied Joel
so he can look into that.

Tested on arm-none-eabi; OK for mainline?

Janis