Let default arc_c_max be platform dependent

Linux changed the default max ARC size to 1/2 of physical memory to
deal with shortcomings of the Linux SLUB allocator.  Other platforms
do not require the same logic.

Implement an arc_default_max() function to determine a default max ARC
size in platform code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10155

include/sys/arc.h

@@ -298,6 +298,7 @@ void arc_tempreserve_clear(uint64_t reserve);
 int arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg);
 
 uint64_t arc_all_memory(void);
+uint64_t arc_default_max(uint64_t min, uint64_t allmem);
 uint64_t arc_target_bytes(void);
 void arc_init(void);
 void arc_fini(void);