glibc: install libmvec.so when available

glibc: install libmvec.so when available

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 glibc package (used for the
internal toolchain backend). Note that libraries listed in
GLIBC_LIBS_LIB 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>