aarch64: Fix negative movrel offsets for windows

On windows, the offset for the relocation doesn't get stored in
the relocation itself, but as an unsigned immediate in the opcode.
Therefore, negative offsets has to be handled via a separate sub
instruction, just as on MachO.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/aarch64/asm.S

@@ -82,6 +82,15 @@ ELF     .size   \name, . - \name
         adrp            \rd, \val+(\offset)@PAGE
         add             \rd, \rd, \val+(\offset)@PAGEOFF
     .endif
+#elif CONFIG_PIC && defined(_WIN32)
+    .if \offset < 0
+        adrp            \rd, \val
+        add             \rd, \rd, :lo12:\val
+        sub             \rd, \rd, -(\offset)
+    .else
+        adrp            \rd, \val+(\offset)
+        add             \rd, \rd, :lo12:\val+(\offset)
+    .endif
 #elif CONFIG_PIC
         adrp            \rd, \val+(\offset)
         add             \rd, \rd, :lo12:\val+(\offset)