plat/kvm,xen: Use LDLIBS-y for -lgcc option

plat/kvm,xen: Use LDLIBS-y for -lgcc option

Use `KVM_LDLIBS-y` and `XEN_LDLIBS-y` for the `-lgcc` linker option.
Instead of the custom flags `KVM_LINK_LIBGCC_FLAG` and
`XEN_LINK_LIBGCC_FLAG`.

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
GitHub-Closes: #1525