build(android): add opt-in flag to disable SVE dispatch on Apple Silicon emulators

[farfromrefug]

Summary

Android emulators running on Apple Silicon advertise SVE, SVE2 and friends in HWCAP even though the host CPU implements none of them. KleidiCV, OpenCV's arm64 HAL, reads HWCAP in its static initializers and selects its SVE2 kernels, so the first SVE instruction executed raises SIGILL and the app dies in nativeCrop as soon as a scan is cropped:

signal 4 (SIGILL), code 1 (ILL_ILLOPC)
  #00 libdocument_detector.so  kleidicv::sve2::remap_s16point5<unsigned char>
  #11 libdocument_detector.so  Java_..._nativeCrop

• Add hwcap_sve.cpp, a getauxval interposition that hides the SVE bits so KleidiCV falls back to its NEON implementations. It is compiled in only when building with --gradleArgs=-PdisableSVE, and is a development aid — enabling it for a release would give up KleidiCV's SVE2 code paths on the devices that really support them.
• The flag value is always passed to CMake explicitly, because a value left in the CMake cache would survive the flag being dropped and silently disable SVE in a release build.

Nothing changes for a normal build.

Testing

Both directions checked with real plugin rebuilds on an arm64 emulator (API 36, Apple Silicon host):

Build	CMake cache	getauxval	Crop
ns build android	DISABLE_SVE:BOOL=0	imported from libc	crashes, unchanged
--gradleArgs=-PdisableSVE	DISABLE_SVE:BOOL=1	bound locally, no PLT call left	capture + crop complete, document saved

Note that the CLI reuses the prebuilt plugin aar, so toggling the flag alone rebuilds nothing until a plugin source changes.