If no extra rtld arguments are provided, rtld_argc will be 1 (for
argv[0] and so we are shifting the entire memory range down by a single
pointer. However, unlike argv and envp, auxp's entries are two pointers
in size, not one, and so in this case the source and destination
overlap, meaning simple assignment is UB (C99 6.5.16.1p3). On many
architectures this ends up being harmless as the compiler will emit
double machine word loads and stores, or if it splits them it may still
schedule them such that it works in this case, but our RISC-V baseline
does not include such instructions and LLVM ends up picking a schedule
that copies the second word before the first word, thereby replacing the
first word with a copy of the second word. This results in direct exec
mode segfaulting on RISC-V when given no arguments.

Fix this by using a temporary in the source and let the compiler safely
elide its use.

Fixes: 0fc65b0ab82c ("Make ld-elf.so.1 directly executable.")
MFC after: 1 week