tests: llext: Add RISC-V CB-Type edge case test

All immediates in RISC-V are encoded as two's complement. This commit
adds a test for relocating jumps that utilize the full range of the
immediate, in both positive and negative direction.
To this end, the test uses the compressed b-type (CB) instruction to
branch to its maximum negative (-256) and maximum positive (+254)
targets.
In case of test failure, expect relocating the corresponding llext to
fail.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>