Rework options interface

Users will want to set options from an environment variable or something
like that.  Particular GC implementations will want to expose an
expanded set of options.  For these reasons we make the options
interface a bit more generalized and include parsing.

gc-platform-gnu-linux.c

@@ -3,6 +3,7 @@
 #include <errno.h>
 #include <link.h>
 #include <pthread.h>
+#include <sched.h>
 #include <stdio.h>
 #include <unistd.h>
 
@@ -102,3 +103,10 @@ void gc_platform_visit_global_conservative_roots(void (*f)(uintptr_t start,
   struct visit_data visit_data = { f, heap, data };
   dl_iterate_phdr(visit_roots, &visit_data);
 }
+
+int gc_platform_processor_count(void) {
+  cpu_set_t set;
+  if (sched_getaffinity(0, sizeof (set), &set) != 0)
+    return 1;
+  return CPU_COUNT(&set);
+}