Add Armv7a NO FP -mno-unaligned-access library variants

[simpal01]

This adds the following library:

• armv7a_soft_nofp_strictly_aligned
• armv7a_soft_nofp_strictly_aligned_exn_rtti

[ostannard]

strictly_aligned

I think it would be better to keep this as one word with no underscores, so that it is clearer how these parse. exn_rtti doesn't follow that, but it could be considered two separate features, and a rtti variant without exceptions could exist in future (not that I think it should).

Since we are eventually going to want strict-aligned variants for most architectures, would it make more sense to handle it like we do for exn_rtti and expand the library variants inside add_library_variants_for_cpu, instead of adding more calls to it? That will need an extra parameter for now to only enable the new variants for architectures where they work, but will avoid a lot of duplication in the long run, especially when we add big-endian variants which would double everything again if done in the same way.

[pratlucas]

LGTM.