[BUILD] Hide symbols from linked static libraries

[tqchen]

To avoid downstream mistakenly picking up symbols from static libstdc++_nonshared, always hide symbols from static libraries linked into tvm-ffi shared objects.

This adds a CMake helper for the policy and applies it to tvm_ffi_shared, tvm_ffi_testing, and the optional tvm_ffi_cython target on ELF platforms.

Local validation:

• Configured and built tvm_ffi_shared and tvm_ffi_testing with TVM_FFI_USE_LIBBACKTRACE=OFF.
• Verified generated Ninja link flags include -Wl,--exclude-libs,ALL for libtvm_ffi.so and libtvm_ffi_testing.so.

[gemini-code-assist]

Code Review

This pull request introduces a new CMake function, tvm_ffi_hide_static_linked_lib_symbols, to prevent symbols from static archives linked into shared libraries from being exported on ELF targets. This function is applied to the tvm_ffi_shared, tvm_ffi_testing, and tvm_ffi_cython targets. Feedback suggests restricting the compiler option -Wl,--exclude-libs,ALL to specific operating systems (such as Linux, Android, and BSDs) using CMAKE_SYSTEM_NAME to prevent build failures on non-ELF UNIX platforms (like Solaris or AIX) that use native system linkers.

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