arc: Update elfarcv2 script template

Update ARC's elfarcv2 script template with:

- The .ivt section (Interrupt Vector Table) is mapped at the begining
  of STARTUP_MEMORY when ivtbase_addr is not defined. Previously, it
  was pointing to 0x00.

- MEMORY_FILE is a new emulation paramter and sets the name for the
  linker script file which holds the MEMORY commands required by
  arcv2elfx emulation.

- Four new linker variables are introduced available when arcv2elf emulation is used:
  * __TEXT_REGION_ORIGIN__ Once defined it is setting the text region origin. By default it points to zero.
  * __TEXT_REGION_LENGTH__ Once defined it is setting the text region length. By default it is set to 2M.
  * __DATA_REGION_ORIGIN__ Once defined it is setting the data region origin. By default it is set to 0x80000000.
  * __DATA_REGION_LENGTH__ Once defined it is setting the data region length. By default it is set to 2M.

ld/ChangeLog:

	* scripttempl/elfarcv2.sc: Update script template.

Signed-off-by: Claudiu Zissulescu <claziss@gmail.com>