unix: use conservative GC by default

This commit does two things:

 1. It makes it possible to grow the heap on Linux and MacOS by
    allocating 1GB of virtual memory on startup and then slowly using it
    as necessary, when running out of available heap space.
 2. It switches the default GC to be the conservative GC (previously
    extalloc). This is good for consistency with other platforms that
    all use this same GC.

This makes the extalloc GC unused by default.

src/runtime/runtime_unix.go

@@ -16,6 +16,9 @@ func usleep(usec uint) int
 //export malloc
 func malloc(size uintptr) unsafe.Pointer
 
+//export mmap
+func mmap(addr unsafe.Pointer, length, prot, flags, fd int, offset int) unsafe.Pointer
+
 //export abort
 func abort()