Workaround for a Cmake bug that happens when cross-compiling using Cmake on macOS x86_64 CPUs with target CPU arm64

[shpala]

This is a temporary workaround for Cmake bug that only sets the correct CMAKE_SYSTEM_PROCESSOR for cross-compilation when target CMAKE_SYSTEM_NAME differs from CMAKE_HOST_NAME:
https://gitlab.kitware.com/cmake/cmake/-/issues/25640

Cross-compilation of a project that is linked to SpeeDB on an x86_64 CPU with target CPU as arm64 fails after SpeeDB was built using the following command:
cmake -G "Unix Makefiles" -DCMAKE_SYSTEM_PROCESSOR=arm64 ..
The SpeeDB build itself succeeds, but executable linking with it will return a linking error.
This specific issue only happens when cross-compiling on an macOS machine due to a possible Cmake bug that causes CMAKE_SYSTEM_PROCESSOR to be equal to CMAKE_HOST_SYSTEM_PROCESSOR regardless of the value of CMAKE_OSX_ARCHITECTURES, which in turn causes an issue in SpeeDBs own CMakeLists.txt.
In my case the issue happened when I was building SpeeDB on x86_64 with a CMAKE_OSX_ARCHITECTURES set to "arm64".
Even after manually setting CMAKE_SYSTEM_PROCESSOR with "-DCMAKE_SYSTEM_PROCESSOR=arm64" or even when setting it manually at the top of CMakeLists.txt using set(CMAKE_SYSTEM_PROCESSOR arm64), immediately after the project() call, the CMAKE_SYSTEM_PROCESSOR was reset back to x86_64.
The linking error happens, because due to wrong CMAKE_SYSTEM_PROCESSOR, the variable HAS_ARMV8_CRC gets the wrong value.
This in turn causes Cmake to skip the source append command of "util/crc32c_arm64.cc".

Expected behavior
Successfully compiled executable.

Actual behavior
The following linking error:
Undefined symbols for architecture arm64:
"crc32c_arm64(unsigned int, unsigned char const*, unsigned long)", referenced from:
speedb::crc32c::ExtendARMImpl(unsigned int, char const*, unsigned long) in libspeedb.a(crc32c.cc.o)
"crc32c_runtime_check()", referenced from:
speedb::crc32c::IsFastCrc32Supported() in libspeedb.a(crc32c.cc.o)
__GLOBAL__sub_I_crc32c.cc in libspeedb.a(crc32c.cc.o)
"crc32c_pmull_runtime_check()", referenced from:
speedb::crc32c::IsFastCrc32Supported() in libspeedb.a(crc32c.cc.o)
__GLOBAL__sub_I_crc32c.cc in libspeedb.a(crc32c.cc.o)

nm -C libspeedb.a returns the following:
crc32c.cc.o:
0000000000000dfc s GCC_except_table1
0000000000000e20 s GCC_except_table21
0000000000000e4c s __GLOBAL__sub_I_crc32c.cc
U __Unwind_Resume
U crc32c_arm64(unsigned int, unsigned char const*, unsigned long)
U crc32c_runtime_check()
U crc32c_pmull_runtime_check()
0000000000000860 T speedb::DecodeFixed32(char const*)"

Steps to reproduce the behavior
Cross-compile a project that is linked to SpeeDB on an x86_64 CPU with target CPU as arm64.
cmake -G "Unix Makefiles" -DCMAKE_SYSTEM_PROCESSOR=arm64 ..