libspl: move physmem to sys/systm.h; initialise at libspl_init()

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17861

lib/libzpool/kernel.c

@@ -60,7 +60,6 @@
  * Emulation of kernel services in userland.
  */
 
-uint64_t physmem;
 uint32_t hostid;
 struct utsname hw_utsname;
 
@@ -509,8 +508,6 @@ kernel_init(int mode)
 
 	umem_nofail_callback(umem_out_of_memory);
 
-	physmem = sysconf(_SC_PHYS_PAGES);
-
 	dprintf("physmem = %llu pages (%.2f GB)\n", (u_longlong_t)physmem,
 	    (double)physmem * sysconf(_SC_PAGE_SIZE) / (1ULL << 30));