[ARM] Add support for thumb1 pcrop relocations.

[ARM] Add support for thumb1 pcrop relocations.

To support thumb1 execute-only code we need to support four new
relocations (R_ARM_THM_ALU_ABS_G0_NC, R_ARM_THM_ALU_ABS_G1_NC,
R_ARM_THM_ALU_ABS_G2_NC and  R_ARM_THM_ALU_ABS_G3_NC).
These relocations allow the static linker to finalize construction
of symbol address.
Typical sequence of code to get address of the symbol foo is then
the following :
	movs	r3, #:upper8_15:#foo
	lsls	r3, #8
	adds	r3, #:upper0_7:#foo
	lsls	r3, #8
	adds	r3, #:lower8_15:#foo
	lsls	r3, #8
	adds	r3, #:lower0_7:#foo
This will give following sequence of text and relocations after
assembly :
   4:	2300      	movs	r3, #0
			4: R_ARM_THM_ALU_ABS_G3_NC	foo
   6:	021b      	lsls	r3, r3, #8
   8:	3300      	adds	r3, #0
			8: R_ARM_THM_ALU_ABS_G2_NC	foo
   a:	021b      	lsls	r3, r3, #8
   c:	3300      	adds	r3, #0
			c: R_ARM_THM_ALU_ABS_G1_NC	foo
   e:	021b      	lsls	r3, r3, #8
  10:	3300      	adds	r3, #0
			10: R_ARM_THM_ALU_ABS_G0_NC	foo