Libatomic: Define per-file identifier macros

In order to facilitate the fine-tuning of how `libatomic_i.h' and
`host-config.h' headers are used by different atomic functions, we
define distinct identifier macros for each file which, in implementing
atomic operations, imports these headers.

The idea is that different parts of these headers could then be
conditionally defined depending on the macros set by the file that
`#include'd them.

Given how it is possible that some file names are generic enough that
using them as-is for macro names (e.g. flag.c -> FLAG) may potentially
lead to name clashes with other macros, all file names first have LAT_
prepended to them such that, for example, flag.c is assigned the
LAT_FLAG macro.

Libatomic/ChangeLog:

	* cas_n.c (LAT_CAS_N): New.
	* exch_n.c (LAT_EXCH_N): Likewise.
	* fadd_n.c (LAT_FADD_N): Likewise.
	* fand_n.c (LAT_FAND_N): Likewise.
	* fence.c (LAT_FENCE): Likewise.
	* fenv.c (LAT_FENV): Likewise.
	* fior_n.c (LAT_FIOR_N): Likewise.
	* flag.c (LAT_FLAG): Likewise.
	* fnand_n.c (LAT_FNAND_N): Likewise.
	* fop_n.c (LAT_FOP_N): Likewise
	* fsub_n.c (LAT_FSUB_N): Likewise.
	* fxor_n.c (LAT_FXOR_N): Likewise.
	* gcas.c (LAT_GCAS): Likewise.
	* gexch.c (LAT_GEXCH): Likewise.
	* glfree.c (LAT_GLFREE): Likewise.
	* gload.c (LAT_GLOAD): Likewise.
	* gstore.c (LAT_GSTORE): Likewise.
	* load_n.c (LAT_LOAD_N): Likewise.
	* store_n.c (LAT_STORE_N): Likewise.
	* tas_n.c (LAT_TAS_N): Likewise.