disable s390x vector intrinsics if softfloat is enabled

[fneddy]

we will add an explicit incompatibility of softfloat and vector feature in rutsc s390x-unknown-none-softfloat target specification. Therefore we need to disable vector intrinsics here to be able to compile core for this target.

=> rust-lang/rust#151154

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[folkertdev]

cc @uweigand you're probably following along already, just making sure you see this.

All of the vector types/functions are unstable currently, but them not being available with softfloat is something we should bring up in an eventual stabilization report.

[uweigand]

cc @uweigand you're probably following along already, just making sure you see this.

All of the vector types/functions are unstable currently, but them not being available with softfloat is something we should bring up in an eventual stabilization report.

Agreed. Just for context, with GCC and clang we have the same situation where vector types and intrinsics are not available with softfloat.