llext: invalidate instruction cache after loading extension

In addition to flushing the dcache, to ensure that the new code is
actually read by the CPU when loading an extension, the icache must
be invalidated as well.

Without this, some tests are failing in hardware because the CPU is
executing stale code from its instruction cache.

Fixes #70906 on arduino_giga_r1/stm32h747xx/m7.

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