RISC-V: Use an autoconf template to produce `preconfigure'

Avoid fiddling with autoconf internals and use AC_DEFINE_UNQUOTED to
define macros in the configuration headers rather than handcoding an
equivalent shell sequence with the use of the `as_echo' undocumented
variable.

Switch to using AC_MSG_ERROR rather than `echo' and `exit' directly for
error handling.  Owing to the lack of any kind of error annotation it
makes it difficult to spot the message in the flood in a parallel build
and neither it is logged in `config.log'.

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>