toolchain-external: install libmvec.so from glibc toolchains

toolchain-external: install libmvec.so from glibc toolchains

On some architectures (namely x86-64), glibc may provide a libmvec
library since glibc 2.22, which programs built with gcc OpenMP support
might get linked to.

In order for these programs to work on the target, we need to copy
this library to the target filesystem.

This commit takes care of this for the external toolchain
situation. Note that libraries listed in TOOLCHAIN_EXTERNAL_LIBS are
silently ignored if they don't exist. Therefore, we don't need to have
any condition on the architecture or glibc version.

For more details on libmvec, see
https://sourceware.org/glibc/wiki/libmvec.

Fixes bug #9111.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>