riscv: alternative: fix typo in macro name

alternative-macros.h defines ALT_NEW_CONTENT in its assembly part
and ALT_NEW_CONSTENT in the C part. Most likely it is the latter
that is wrong.

Fixes: 6f4eea90


	(riscv: Introduce alternative mechanism to apply errata solution)
Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>