package/tremor: fix build with Thumb only ARM archs

package/tremor: fix build with Thumb only ARM archs

Some ARM architectures like ARMv7-M only supports Thumb instructions,
but the tremor build configuration enables ARM assembly code
unconditionally for all arm triplets by defining _ARM_ASSEM_.

We are overriding this by undefining this macro for ARM architectures
not supporting ARM instructions.

Fixes:
http://autobuild.buildroot.net/results/054/054f1c77b0e5d46b2dc53769469c0ed03e6b79ef/
http://autobuild.buildroot.net/results/ba1/ba1760b1428584f70e44dbffb8218ff3ee55e702/
http://autobuild.buildroot.net/results/2a6/2a687853cf0bc832fef29f88de0d85bd495fe87d/
http://autobuild.buildroot.net/results/cb6/cb6c560bf31834aadbe3d13a118b31ea8190159b/

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>