linux.config: Fix cross compilation with autotools
Older versions of GStreamer have a configure.ac which uses a test program
for check if unaligned access is possible. Because this test can not be run
when cross-compiling, the safer or more conservative value for this is
zero. This test it's been used for set the macro
GST_HAVE_UNALIGNED_ACCESS
which chooses between faster and slower
implementations for read/swap integers from a memory buffer.