[builtins] Support architectures with 16-bit int in __ashlti3, __ashrti3 and __lshrti3

The amount to shift should be specified by the int type not a 32-bit integer
type.

This patch change the functions for 128-bit shifts in compiler-rt the same way
as was done for 64-bit shifts in D78662.

The README.txt is updated with the shift builtins signatures from this patch and D78662.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D146960