check for and use C library provided strlcpy and strlcat

The musl-libc library provides implementations of strlcpy and strlcat,
so introduce a feature check for it and only use the kvmtool
implementation if there is no library support for it.
This avoids clashes with the public definition.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>