arm64: idreg-override: Avoid sprintf() for simple string concatenation

arm64: idreg-override: Avoid sprintf() for simple string concatenation

Instead of using sprintf() with the "%s.%s=" format, where the first
string argument is always the same in the inner loop of match_options(),
use simple memcpy() for string concatenation, and move the first copy to
the outer loop. This removes the dependency on sprintf(), which will be
difficult to fulfil when we move this code into the early mini C
runtime.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231129111555.3594833-61-ardb@google.com
Signed-off-by: Will Deacon <will@kernel.org>