build: Fixes for non-SSE x86

[willnode]

This fixes compilation for non-SSE x86 after commenting out assertions.

This is the cherrypick of what I send to fix redox i586 build after rebase: https://gitlab.redox-os.org/redox-os/ring/-/merge_requests/5

The change in x86.rs is because leaf1_edx is already exist in the function

The change in prefixed.rs is because in src/arithmetic/limbs/fallback/mont.rs the macro is used for non-SSE2 x86

 if #[cfg(not(any(
            all(target_arch = "aarch64", target_endian = "little"),
            all(target_arch = "arm", target_endian = "little"),
            all(target_arch = "x86", target_feature = "sse2"),
            target_arch = "x86_64")))] {
        // this is called if all(target_arch = "x86", not(target_feature = "sse2"))
        prefixed_export! {
            #[cfg_attr(target_arch = "x86", cold)]
            #[cfg_attr(target_arch = "x86", inline(never))]
            unsafe extern "C" fn bn_mul_mont_fallback(

[willnode]

Related issue #1999 i opened this also because more change was needed to fix compilation for targets without SSE