Linux 6.7 compat: detect if kernel defines intptr_t

Since Linux 6.7 the kernel has defined intptr_t. Clang has
-Wtypedef-redefinition by default, which causes the build to fail
because we also have a typedef for intptr_t.

Since its better to use the kernel's if it exists, detect it and skip
our own.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16201