Fix testcases using __attribute__((noclone)) with Clang

Fix testcases using __attribute__((noclone)) with Clang

Clang fails to compile a number of files with the following warning:
unknown attribute 'noclone' ignored [-Wunknown-attributes].  This
commit adds a new header, lib/noclone.h, which defines the macro
ATTRIBUTE_NOCLONE accordingly, and updates the relevant testcases
to use it.

gdb/testsuite/ChangeLog:

	* lib/attributes.h: New header.
	* gdb.base/backtrace.c: Include the above. Replace
	__attribute__(noclone)) with ATTRIBUTE_NOCLONE.
	* gdb.base/infcall-nested-structs.c: Likewise.
	* gdb.base/vla-optimized-out.c: Likewise.