all: add the Boehm-Demers-Weiser GC on Linux

This adds support for the well-known Boehm GC. It's significantly faster
than our own naive GC and could be used as an alternative on bigger
systems.

In the future, this GC might also be supported on WebAssembly with some
extra work. Right now it's Linux only (though Windows/MacOS shouldn't be
too difficult to add).

src/runtime/gc_blocks.go

@@ -534,6 +534,12 @@ func markRoots(start, end uintptr) {
 	}
 }
 
+func markCurrentGoroutineStack(sp uintptr) {
+	// This could be optimized by only marking the stack area that's currently
+	// in use.
+	markRoot(0, sp)
+}
+
 // stackOverflow is a flag which is set when the GC scans too deep while marking.
 // After it is set, all marked allocations must be re-scanned.
 var stackOverflow bool