[AArch64] Simplify frame pointer logic

[AArch64] Simplify frame pointer logic

Simplify frame pointer logic.  Add aarch64_needs_frame_chain to
decide when to emit the frame chain using clearer logic.
Introduce aarch64_use_frame_pointer which contains the value of
-fno-omit-frame-pointer (flag_omit_frame_pointer is set to a magic
value so that the mid-end won't force the frame pointer in all
cases, and leaf frame pointer emission can't be supported).

    gcc/
	* config/aarch64/aarch64.c (aarch64_use_frame_pointer):
	Add new boolean.
	(aarch64_needs_frame_chain): New function.
	(aarch64_parse_override_string): Set aarch64_use_frame_pointer.

From-SVN: r260606