Private libc is not initialized on Ubuntu 21

This was discovered in DynamoRIO/drmemory#2425

The upshot is that in glibc-2.32-403, ld.so now calls a secret routine __libc_early_init to initialize some key pieces of libc, rather than using regular ELF constructors as in the past. This can easily result in tool crashes referencing NULL pointers. We'll have to mirror this hardcoded special behavior in DR's loader.