Add System/390 support, etc.
- IBM System/390 support is added (PR #291)
- The library can be built with Clang on Windows (PR #300)
- Static libraries with LTO can be generated (PR #290)
- Alternative division and sqrt methods can be chosen with AArch64 (PR #289)
- Header files for inlining the whole SLEEF functions can be generated (PR #283)
- IEEE remainder function is added (PR #271)
- GCC-10 can now build SLEEF with SVE support (PR #310)