Add variadic logging to libdebuggerd internal.

GWP-ASan's crash information retrieval services requires a Printf()
function (declared by the system/implementing allocator). In this
instance, because _LOG is called with additional arguments (the log_t),
this function must be wrapped to conform to printf_t defined by
GWP-ASan.

We can easily wrap the variadic version.

Bug: 135634846
Test: atest debuggerd_test
Change-Id: I17209cd2b7455ce889e2f8194969f606cac329eb