SSE, XMM and GMP's configure

[Torbjorn Granlund]

For what it is worth, there are now 32-bit and 64-bit fbsd5 and fbsd56
systems in the test array:  https://gmplib.org/devel/testsystems.html

Both seem to allow XMM access.  The problem might be limited to fbsd4.

At some point, it would be nice to clean up the broken logics for this
in GMP's configure, but we should keep it mind that it is not very
important in practice.  (We could actually consider fixing it, or
removing the XMM checking code altogether.)