primesieve-12.7

This release adds ARM SVE runtime dispatching, it is enabled by default on Linux for ARM64 CPUs and it has also been implemented for Windows (but support in Microsoft's Windows.h is still missing). The C/C++ API and ABI of this release are fully backwards compatible with primesieve-12.*

• multiarch_sve_arm.cmake: Improve ARM SVE detection.
• src/arch/arm/sve.cpp: Detect ARM SVE on Linux and Windows.
• EratBig.cpp: Simplify bucket handling.
• Erat.cpp: Tune sieve size using FACTOR_SIEVESIZE.
• README.md: Add Sponsors section.

Thanks to @AndrewVSutherland and @AlgoWin for being primesieve sponsors in this release cycle!