[mask_rom] Move extern declarations closer to uses

Make the mask ROM assembly more consistent by moving extern
declarations closer to their uses. Purely a cosmetic change.

Signed-off-by: Michael Munday <mike.munday@lowrisc.org>
diff --git a/sw/device/silicon_creator/mask_rom/mask_rom_start.S b/sw/device/silicon_creator/mask_rom/mask_rom_start.S
index 0bc9170..b777bf8 100644
--- a/sw/device/silicon_creator/mask_rom/mask_rom_start.S
+++ b/sw/device/silicon_creator/mask_rom/mask_rom_start.S
@@ -230,11 +230,8 @@
   li x30, 0x0
   li x31, 0x0
 
-  .extern crt_section_clear
-  .extern crt_section_copy
-  .extern mask_rom_epmp_init
-
   // Must be called prior to any Main RAM access.
+  .extern mask_rom_epmp_init
   call mask_rom_epmp_init
 
   /**
@@ -245,11 +242,13 @@
   la   a0, _data_start
   la   a1, _data_end
   la   a2, _data_init_start
+  .extern crt_section_copy
   call crt_section_copy
 
   // Initialize the `.bss` section.
   la   a0, _bss_start
   la   a1, _bss_end
+  .extern crt_section_clear
   call crt_section_clear
 
   // Setup global pointer.