Silence objtool warnings from 55d7afa4

The use of noreturn in 55d7afa4adbb4ca569e9c4477a7d121f4dc0bfbd on
spl_panic() caused objtool warnings on Linux when the kernel is built
with CONFIG_STACK_VALIDATION=y. This patch works around that by
restricting the application of noreturn to builds for static
analyzers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14068