bfd: xtensa: ignore overflow in hight part of const16 relocation

bfd: xtensa: ignore overflow in hight part of const16 relocation

32-bit constants loaded by two const16 opcodes that involve relocation
(e.g. calculated as a sum of a symbol and a constant) may overflow,
resulting in linking error with the following message:

  dangerous relocation: const16: cannot encode: (_start+0x70000000)

They should wrap around instead. Limit const16 opcode immediate field to
16 least significant bits to implement this wrap around.

bfd/
2018-12-11  Max Filippov  <jcmvbkbc@gmail.com>

	* elf32-xtensa.c (elf_xtensa_do_reloc): Limit const16 opcode
	immediate field to 16 least significant bits.