include: arch: arm: cortex_m: Provide __rom_region_size linker symbol

__rom_region_size linker symbol is available for other archs, but was
missing for Cortex-M. Add it now.

Signed-off-by: Erwan Gouriou <[email protected]>

Author: erwango

include/zephyr/arch/arm/cortex_m/scripts/linker.ld

@@ -241,6 +241,7 @@ SECTIONS
 	MPU_ALIGN(__rodata_region_end - ADDR(rom_start));
 #endif
 	__rom_region_end = __rom_region_start + . - ADDR(rom_start);
+	__rom_region_size = __rom_region_end - __rom_region_start;
 
     GROUP_END(ROMABLE_REGION)