toolchain-external: improve musl external check

toolchain-external: improve musl external check

The current test to verify if the toolchain uses musl or not is based on
checking if /lib/libc.so or /lib/libm.so exist in the sysroot. However,
some toolchains (notably Crosstool-NG ones) put these libraries in
/usr/lib/.

To fix this, build a minimal C program and check if the program
interpreter contains /lib/ld-musl.

Signed-off-by: Ilya Kuzmich <ilya.kuzmich@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>