tests/tcg/x86_64/fma: Test some x86 fused-multiply-add cases

tests/tcg/x86_64/fma: Test some x86 fused-multiply-add cases

Add a test case which tests some corner case behaviour of
fused-multiply-add on x86:
 * 0 * Inf + SNaN should raise Invalid
 * 0 * Inf + QNaN shouldh not raise Invalid
 * tininess should be detected after rounding

There is also one currently-disabled test case:
 * flush-to-zero should be done after rounding

This is disabled because QEMU's emulation currently does this
incorrectly (and so would fail the test).  The test case is kept in
but disabled, as the justification for why the test running harness
has support for testing both with and without FTZ set.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/r/20250116112536.4117889-3-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>