Define MAP_ANONYMOUS if not defined

[ryandesign]

Building abseil on OS X 10.10 and earlier fails:

absl/debugging/internal/examine_stack.cc:58:34: error: use of undeclared identifier 'MAP_ANONYMOUS'
                   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
                                 ^
1 error generated.

The fix for this issue is to define MAP_ANONYMOUS to be equal to the older MAP_ANON if it is not already defined.

There are other places in the abseil code where this was already being done (see #120), just not everywhere.

In failure_signal_handler.cc, the fix was embedded in the middle of a function. It seems cleaner to have it near the top of the file, as it is in low_level_alloc.cc, so I added it to the top in the new files and moved it to the top in failure_signal_handler.cc.

In low_level_alloc.cc, the fix was restricted only to Apple platforms, and this had to be expanded to a second platform recently (see be85b34):

abseil-cpp/absl/base/internal/low_level_alloc.cc

Lines 60 to 67 in be85b34

	// MAP_ANONYMOUS
	#if defined(__APPLE__) || defined(__hexagon__)
	// For mmap, Linux defines both MAP_ANONYMOUS and MAP_ANON and says MAP_ANON is
	// deprecated. In Darwin, MAP_ANON is all there is.
	#if !defined MAP_ANONYMOUS
	#define MAP_ANONYMOUS MAP_ANON
	#endif // !MAP_ANONYMOUS
	#endif // __APPLE__

There's no reason to restrict it by platform at all; just define it if it's not defined, on any platform. The comment about MAP_ANONYMOUS not existing on Darwin is also outdated; it does exist in Darwin on OS X 10.11 (released 2015) and later. So I've also removed these comments and restrictions and made this block the same as the others.

Maybe there is some central header where this could be addressed once instead of being copied into each file that uses MAP_ANONYMOUS but I don't know the abseil code well enough to determine that.

Feel free to edit the code or commit message as required for your standards.