PR71951: Fix unwinding with -fomit-frame-pointer

PR71951: Fix unwinding with -fomit-frame-pointer

As described in PR71951, if libgcc is built with -fomit-frame-pointer,
unwinding crashes, for example while doing a backtrace.  The underlying
reason is the Dwarf unwinder does not setup the frame pointer register
in the initialization code.  When later unwinding a function that uses
the frame pointer, it tries to read FP using _Unwind_GetGR, and this
crashes if has never restored FP.  To unwind correctly the first frame
must save and restore FP (it is unwound in a special way so that it
uses SP instead of FP).  This is done by adding -fno-omit-frame-pointer.

    gcc/
	PR target/71951
	* config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@253061 138bc75d-0d04-0410-961f-82ee72b054a4