[MC] Widen the functional unit type from 32 to 64 bits.

Summary:
The type used to represent functional units in MC is
'unsigned', which is 32 bits wide. This is currently
not a problem in any upstream target as no one seems
to have hit the limit on this yet, but in our
downstream one, we need to define more than 32
functional units.

Increasing the size does not seem to cause a huge
size increase in the binary (an llc debug build went
from 1366497672 to 1366523984, a difference of 26k),
so perhaps it would be acceptable to have this patch
applied upstream as well.

Subscribers: hiraditya, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71210