arm-bsp/musca-*: use newlib for libc

Whilst glibc is definitely not an option for Cortex-M cores, newlib is
a viable libc implementation that is useful to default too.

Change-Id: I1f3a53e43945f486aa2e7bce019f105c55df5e87
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>