editors/libreoffice: fix i386/powerpc libcxxrt ABI change detection

This makes the bundled bridgetest_server/bridgetest_client working.

Recently the size of struct __cxa_exception changed on 32 bit
architectures. A patch committed in 35a5054819f1 lets libreoffice
detect this by comparing a field of the struct against a known value.
In the bridgetest it turns out this value isn't always the same, so use
a different field instead.

PR: 263596