src/sys: dynamically detect FZ/DAZ support

With binary builds of Julia becoming more common, it's better to dynamically
detect whether the processor supports SSE/SSE2 and choose the flags based
on that.

For non-x86/x86-64 processors, a stub function is supplied.
test/corelib: add test for denormal flags

Because the dynamic-SSE-test version of jl_zero_denormals uses inline assembler,
and to make sure the arch-specific stuff is done correctly, add a couple of
tests just to make sure things aren't crashing.
Merge branch 'topic/dynamic-zebras'