Svelter libart-compiler

Added new environment variable ART_{TARGET,HOST}_CODEGEN_ARCHS which
may be set to 'all', 'svelte' or a space separated list of architectures.

When compiled with ART_{TARGET,HOST}_CODEGEN_ARCHS='all' (the default
value) dex2oat will be able to generate output for all supported
architectures.

When compiled with ART_TARGET_CODEGEN_ARCHS='svelte'
only the architectures of the TARGET will be included. When
ART_HOST_CODEGEN_ARCHS='svelte' all architectures the target includes
and the host architectures will be included on the host dex2oat.

If a list of architectures is given only those will be included.

Change-Id: I87f4ad0131ab1b37544d8799e947ce4733b6daec