toolchain-external: use BR2_HOSTARCH_NEEDS_IA32_LIBS where needed

All supported pre-built external toolchains are built for x86 Linux,
so we add the BR2_HOSTARCH_NEEDS_IA32_LIBS select.

[Peter: microblaze toolchains are 64bit]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>