[mlir] Use MCJIT to fix integration tests

Since c42c67ad ('Re-apply "[lli] Make -jit-kind=orc the default JIT
engine"'), ORC is the default JIT. Unfortunately, ORC seems to
ignore the --entry-function flag, which breaks all tests that
use the flag, namely the AMX and X86Vector integration tests.
This has been reported in PR#49906
(https://bugs.llvm.org/show_bug.cgi?id=49906).

Work around this by explicitly selecting MCJIT.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D100344