tsan: Add optional support for distinguishing volatiles

tsan: Add optional support for distinguishing volatiles

Add support to optionally emit different instrumentation for accesses to
volatile variables. While the default TSAN runtime likely will never
require this feature, other runtimes for different environments that
have subtly different memory models or assumptions may require
distinguishing volatiles.

One such environment are OS kernels, where volatile is still used in
various places, and often declare volatile to be appropriate even in
multi-threaded contexts. One such example is the Linux kernel, which
implements various synchronization primitives using volatile
(READ_ONCE(), WRITE_ONCE()).

Here the Kernel Concurrency Sanitizer (KCSAN), is a runtime that uses
TSAN instrumentation but otherwise implements a very different approach
to race detection from TSAN:

	https://github.com/google/ktsan/wiki/KCSAN

Due to recent changes in requirements by the Linux kernel, KCSAN
requires that the compiler supports tsan-distinguish-volatile (among
several new requirements):

	https://lore.kernel.org/lkml/20200521142047.169334-7-elver@google.com/

gcc/
	* params.opt: Define --param=tsan-distinguish-volatile=[0,1].
	* sanitizer.def (BUILT_IN_TSAN_VOLATILE_READ1): Define new
	builtin for volatile instrumentation of reads/writes.
	(BUILT_IN_TSAN_VOLATILE_READ2): Likewise.
	(BUILT_IN_TSAN_VOLATILE_READ4): Likewise.
	(BUILT_IN_TSAN_VOLATILE_READ8): Likewise.
	(BUILT_IN_TSAN_VOLATILE_READ16): Likewise.
	(BUILT_IN_TSAN_VOLATILE_WRITE1): Likewise.
	(BUILT_IN_TSAN_VOLATILE_WRITE2): Likewise.
	(BUILT_IN_TSAN_VOLATILE_WRITE4): Likewise.
	(BUILT_IN_TSAN_VOLATILE_WRITE8): Likewise.
	(BUILT_IN_TSAN_VOLATILE_WRITE16): Likewise.
	* tsan.c (get_memory_access_decl): Argument if access is
	volatile. If param tsan-distinguish-volatile is non-zero, and
	access if volatile, return volatile instrumentation decl.
	(instrument_expr): Check if access is volatile.

gcc/testsuite/
	* c-c++-common/tsan/volatile.c: New test.