Most microprocessors have efficient ways to perform CRC operations, be
that with lookup tables, rotates, or even special instructions.
However, because we lack a representation for CRC in the compiler, we
can't do proper instruction selection.  With this patch I seek out to
rectify this,
I've avoided using a mode name for the built-in functions because that
would tie the semantics to the size of the addressable unit.  We
generally use abbreviations like s/l/ll for type names, which is all
right when the type can be widened without changing semantics.  For
the data input, however, we also have to consider the shift count that
is tied to it.  That is why I used a number to designate the width of
the data input and shift.

For machine support, I made a start with 8 and 16 bit little-endian
CRC for RISCV using a
lookup table.  I am sure once we have the basic infrastructure in the
tree, we'll get more
contributions of suitable named patterns for various ports.

bootstrapped on x86_64-pc-linux-gnu .