grub: force -m32 so we build grub 32 bits on 64 bits architecture

As reported by Matthew Weber, Grub doesn't build on 64 bits build
machines, because the host compiler produces 64 bits binaries by
default, while Grub should be built 32 bits. Therefore, this commit
passes -m32 to the Grub CFLAGS, so that 32 bits binaries are always
produced.

Reported-by: Matthew Weber <mlweber1@rockwellcollins.com>
Cc: Matthew Weber <mlweber1@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Matthew Weber <mlweber1@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>