lib/syscall_shim: Add syscall alias to uk_syscall

The symbol is required for binaries that were compiled against a Linux
musl and use musl's system call wrapper.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>

lib/syscall_shim/uk_syscall.awk

@@ -56,4 +56,5 @@ END {
 	printf "\tva_end(ap);\n"
 	printf "\treturn ret;\n"
 	printf "}\n"
+	printf "long syscall() __attribute__ ((weak, alias (\"uk_syscall\")));\n"
 }