ARC: Recognize mov_s fp,sp instruction in prologue

Most simple prologue must have two instructions:

	push fp ; disassembled as st.a fp,[sp,-4]
	mov fp,sp

For ISA v2 "mov" (major opcode 0x04) in prologue is replaced by "mov_s"
(major opcode 0x08). With this patch GDB can properly skip prologue in
applications for ARC v2. Compatibility with ARC v1 is retained.

Signed-off-by: Anton Kolesov <akolesov@synopsys.com>