[mxcpp][mxalloc] Split mxcpp into mxcpp and mxalloc

mxcpp used to provide three things.

(1) is a very minimal C++ runtime environment. This means essentially
the definition of __cxa_pure_virtual, which is a function the compiler
expects to have ambient. This functionality is necessary for code
written in environments which cannot link the standard library (the
kernel and a handful of initial userspace processes).

(2) is the definitions of the standard new and delete
overloads. Again, these are needed for code that wants to call them
but to not link the standard library, but this time only in userspace
(the kernel does not want certain behavior stemming from the standard
overloads). The behavior is slightly different than the standard ones
in failure cases, but not in a way particularly interesting to the
rest of this email.

(3) is the userspace version of our special AllocChecker new
overloads. This mechanism exists for kernelspace, where allocation
failure is real and needs to be handled. This mechanism works
perfectly well in userspace, in that it compiles, even though the
underlying allocator is unlikely to ever return nullptr. It's
therefore essentially harmless in userspace. A handful of small
libraries are used in both userspace and in the kernel, and also use
this mechanism. Note that this feature is essentially copy-pasted
between the kernel and userspace.

This all works perfectly well for the kernel.

Some userspace code also uses these mechanisms, as mentioned. As long
as the standard library isn't linked, there are again no issues.

But as soon as you want both (3) and std::vector, which is a
reasonable thing to want!, there are issues because (1) and (3) come
in the same static library. This causes link failures due to duplicate
__cxa_pure_virtual symbols. This issue is exacerbated by different
behavior in Linux, OS X, and target linkage, in terms of confusion and
it-works-on-my-machine sort of issues.

This patch splits mxcpp into: mxcpp, containing (1) __cxa_pure_virtual
and (2) only if in userspace, the standard new and delete overloads;
and mxalloc, containing (3) AllocChecker. mxcpp is mutually exclusive
of the standard library. mxalloc can be linked against both code using
the standard library, and against code using mxcpp.

This patch also deduplicates the AllocChecker between userspace and
kernel, and moves the InlineArray class into the kernel (its only
callsites are in the kernel).

Change-Id: Id42d5d019ee977d44d6cb89ce7ab6884f7a56cd1

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5

[magenta] Initial commit