bootstrap: Compile with _FILE_OFFSET_BITS=64 on Linux

To avoid problems accessing filesystem, explicitly use 64-bit file
offsets in case the compilation model is 32-bit.  This change was made
for the main CMake build by commit 5b10f96793 (Linux: Compile with
_FILE_OFFSET_BITS=64 on 32-bit Linux, 2020-09-23, v3.19.0-rc1~112^2),
but conditionally on 32-bit Linux.  For bootstrap, just always add the
definition instead of trying to detect the architecture.

Fixes: #22328