Zupt v2.1.1: fix cross-arch build issues, eliminate UB, improve Android/Termux support

- Removed all shipped .o files from tarball (fixes aarch64/Termux linker errors with x86_64 objects)
- Added arch-safety guard in Makefile to auto-detect and remove incompatible .o files
- Switched default compiler from gcc to cc (Termux uses clang)
- Skipped -lpthread on Android (bionic provides pthreads)
- Added Android detection via uname -o
- Fixed Keccak UB: ROL64(x,0) no longer expands to undefined x >> 64
- Achieved zero UBSan/ASAN issues across all PQ crypto paths
- Moved sys/syscall.h include to file scope with proper __linux__ guard

Release stats:
- 73 files, 159KB, zero .o artifacts
- 70/70 tests passing
- Fully clean under ASAN + UBSan

Note: full-disk encryption (--disk) deferred to v2.2.0 (requires raw device I/O, sparse detection, and privilege handling)

include/zupt.h

@@ -30,7 +30,7 @@
   #define zupt_mkdir(p) mkdir(p, 0755)
 #endif
 
-#define ZUPT_VERSION_STRING "2.1.0"
+#define ZUPT_VERSION_STRING "2.1.1"
 #define ZUPT_FORMAT_MAJOR   1
 #define ZUPT_FORMAT_MINOR   4