Acquire and release the TSAN synchronization point when calling malloc,
just as we do when calling any other C function. If we don't do this,
TSAN will report false positive errors about races calling malloc and
free.

We used to have a special code path for malloc and free, going through
the runtime functions cmalloc and cfree. The special code path for cfree
was no longer used even before this CL. This CL stops using the special
code path for malloc, because there is no place along that path where we
could conditionally insert the TSAN synchronization. This CL removes
the support for the special code path for both functions.

Instead, cgo now automatically generates the malloc function as though
it were referenced as C.malloc.  We need to automatically generate it
even if C.malloc is not called, even if malloc and size_t are not
declared, to support cgo-provided functions like C.CString.

Change-Id: I829854ec0787a80f33fa0a8a0dc2ee1d617830e2
Reviewed-on: https://go-review.googlesource.com/23260Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>