ispc: do not test on the "host" target

This is because some very sensitive tests may fail when using more
exotic platforms and vector instructions.  For example, floating point
summation (which is not associative in the first place), or
transcendental functions.  Arguably, ispc's test suite is too
sensitive here.

(cherry picked from commit 12f03a0)
Backport of NixOS#70260

pkgs/development/compilers/ispc/default.nix

@@ -1,5 +1,5 @@
 {stdenv, fetchFromGitHub, which, m4, python, bison, flex, llvmPackages,
-testedTargets ? ["sse2" "host"] # the default test target is sse4, but that is not supported by all Hydra agents
+testedTargets ? ["sse2"] # the default test target is sse4, but that is not supported by all Hydra agents
 }:
 
 stdenv.mkDerivation rec {