x86: Refine the way we configure the x86 build.

This is the result of a recent conversation around the x86 ABI
and its relationship with how we build platform images. Briefly:

- We want to use feature-based macros like ARCH_X86_HAVE_$FEATURE
  instead of relying on explicit compiler macros like -DUSE_$FEATURE

- We want to allow for other x86-based arch variants, e.g. if
  someone wants to build Android for a CPU that doesn't support
  the NDK x86 ABI. However, we need to ensure these are not
  tagged compatible by mistake (see check at the end of TARGET_linux-x86.mk)

- There are several flags which use is dubious now that we have
  a dedicated toolchain to build all the x86 stuff. Comment them
  as 'to be considered for removal'. We'll do the proper checks
  later.

Change-Id: I7768d7c34d73e274cbf4c09ae831b55280d6bb58

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