dependencies.sh: stop stray a.out being created for ia32 compiler test

If the grub package is selected it also selects
BR2_HOSTARCH_NEEDS_IA32_COMPILER.  This triggers a test in dependencies.sh
to verify the host compiler can build 32 bit executables.  Currently this
test does not set any output for the compiler which causes a stray a.out
to be create outside the output directory.  This patch sets the compiler
output to /dev/null so no a.out is created but the test is still performed.

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>