[TableGen] Fix warning when compiling generated MCCodeEmitter

This fixes an instance of:
warning: cast from 'const unsigned long *' to 'unsigned char *' drops const qualifier [-Wcast-qual]
when compiling the generated MCCodeEmitter for an out-of-tree target
that uses the optional support for instruction widths > 64 bits.

Differential Revision: https://reviews.llvm.org/D97942