toolchain-external: fix musl external support on x86

On x86, the symbolic link ld-musl-<ARCH>.so.1 to libc.so must be
ld-musl-i386.so.1 in all cases, but $(ARCH) in Buildroot might be
i386, i486, i586, i686, etc. depending on the specific x86 variants
being selected.

This commit fixes that by creating a MUSL_ARCH variable set to i386 on
x86, and to $(ARCH) on other architectures.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>