runtime: implement MemStats.NumGC

Reading runtime.MemStats.NumGC is common enough in dependency code that its
absence is a compile error for programs that never look at the value. Rather
than add the field as a constant zero, track it:

- gc_blocks: count completed cycles in runGC, so collections triggered by an
  allocation are counted as well as explicit runtime.GC() calls. The counter is
  read and written under gcLock, like the other counters beside it.
- gc_boehm: report bdwgc's own gc_no from the prof_stats struct.
- gc_leaking: always 0, since that collector never completes a cycle.

builder/sizes_test.go

@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 4313, 323, 0, 2260},
-		{"microbit", "examples/serial", 2838, 382, 8, 2256},
-		{"wioterminal", "examples/pininterrupt", 8027, 1665, 132, 7488},
+		{"hifive1b", "examples/echo", 4321, 323, 0, 2268},
+		{"microbit", "examples/serial", 2842, 382, 8, 2264},
+		{"wioterminal", "examples/pininterrupt", 8039, 1665, 132, 7496},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the