llext: add test for section and symbol alignment

Add a test case for the alignment support in loadable extensions. This
test case creates a set of constants with specific alignment requirements
and verifies that they are placed in memory as expected.

Fix the detached section test to use a more standard syntax for the
section attribute, avoiding issues with different toolchains.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>