user's guide: update docs for X86 CPU flags

This commit updates the section of the user's guide pertaining to
X86 feature flags with the following changes:

  - the NCG backend now supports SIMD, so remove all text
    that says the contrary,
  - the LLVM backend does not "automatically detect" features,
    so remove any text that makes that claim.

(cherry picked from commit 9c9c790d)