For the System V ABI, variadic calls must store the number of floating
point arguments into RAX. This was mostly working by accident for
our calls to printf since RAX is used as a scratch register, and was
often non-zero, which is all that's really needed for printf with float
args to work; however, when RAX would become 0, printf would print the
wrong thing.

Bug: b/149913889
Change-Id: Id4b519c5416927d537fca078109c0dc850f08359
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41668Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>