linker warning "cannot find entry symbol _start" in drbbdup asm tests

The bbdup asm tests, like suite/tests/client-interface/drbbdup-meta-label-bb-test.asm, produce these linker warnings:

[  8%] Linking C executable bin/client.drbbdup-empty-bb-test-exe
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000

They contain main instead of _start.