[PATCH, AArch64, Testsuite] Specify -fno-use-caller-save for func-ret* tests

[Yufeng Zhang <Yufeng dot Zhang at arm dot com>]

Hi,

This patch resolves a conflict between the aapcs64 test framework for 
func-ret tests and the optimization option -fuse-caller-save, which was 
enabled by default at -O1 or above recently.

Basically, the test framework has an inline-assembly based mechanism in 
place which invokes the test facility function right on the return of a 
tested function.  The compiler with -fuse-caller-save is unable to 
identify the unconventional call graph and carries out the optimization 
regardless.

Adding explicit LR clobbering field to the inline assembly doesn't solve 
the issue as the compiler would simply generate extra save/store of LR 
in the prologue/epilogue.

OK for the trunk?

Thanks,
Yufeng

gcc/testsuite/

	* gcc.target/aarch64/aapcs64/aapcs64.exp:
	(additional_flags_for_func_ret): New variable based on $additional_flags
	plus -fno-use-caller-save.
	(func-ret-*.c): Use the new variable.

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