arm: Make sure we can handle a thumb entry point.

Similarly to what's been done on arm64 with commit
712c060c94fd447c91b0e6218c12a431206b487a, when executing a binary, if the
entry point is a thumb symbol, then make sure we set the PSL_T flag, otherwise
the CPU will interpret it in ARM mode, and that will likely leads to an
undefined instruction.

PR: 256899
MFC after: 1 week

(cherry picked from commit 8c3bd133dd52824e427e350c65eae1fd9eb5a3cd)
Signed-off-by: Olivier Houchard <cognet@FreeBSD.org>