devel/llvm16: enable MLIR by default, fix aarch64

Some ports require on MLIR so reenable it by default. This turns it on
for all platforms where it was previously disabled for 32-bit platforms
and pulled in indirectly by FLANG. If this proves to cause too many
user issues we'll disable it for 32-bit again.[0]

Fix a missed 16.0.0->16 change in backend include directories which
broke aarch64 with BE_NATIVE. Also enable the ARM (32-bit) backend on
aarch64 as it's plausiably useful.[1]

PR: 269927 [0]
Reported by: yuri [0], Mark Millard <marklmi@yahoo.com> [1]