This is to conditionally (ifdef) include only the enabled
target assemblers. Also rename the assembler's "x86"
namespace to "X8632" for similar reasons. The namespace was
created to hide generic sounding classes like "Address"
which are used all over the assembler.

Plop the somewhat empty AssemblerARM32 in an ARM32
namespace for consistency.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1114223002