x86: disable `--gc-sections` for Travis' sake

I could not reproduce the problem at home, but on Travis CI after
merging #1415 tests/unittest failed to execute for qemu-i386.

There is a crash early in the initialization, caused by a #PF. The
execution hangs afterwards (`cli; 0: hlt; jmp 0b`), and Travis kills
the execution after 10 minutes.