Export memalign to linker

In order to support demangling of rust symbols by the linker, we are
adding a small Rust component. Rust expects `memalign` to be present in
hosted environments, and it doesn't appear costly to enable it.

Bug: 178565008
Test: m, killall -11 keystore2 produced mangled names in tombstone
Change-Id: I8fc749000fa02a3b760c8cc55be3348b9964d931

linker/linker_memory.cpp

@@ -72,6 +72,10 @@ void* malloc(size_t byte_count) {
   return get_allocator().alloc(byte_count);
 }
 
+void* memalign(size_t alignment, size_t byte_count) {
+  return get_allocator().memalign(alignment, byte_count);
+}
+
 void* calloc(size_t item_count, size_t item_size) {
   return get_allocator().alloc(item_count*item_size);
 }