[RISCV] Correctly report sizes for builtin fixups

This is a different approach to fixing the problem described in D46746. 
RISCVAsmBackend currently depends on the getSize helper function returning the 
number of bytes a fixup may change (note: some other backends have a similar 
helper named getFixupNumKindBytes). As noted in that review, this doesn't 
return the correct size for FK_Data_1, FK_Data_2, or FK_Data_8 meaning that 
too few bytes will be written in the case of FK_Data_8, and there's the 
potential of writing outside the Data array for the smaller fixups.

D46746 extends getSize to recognise some of the builtin fixup types. Rather 
than having a function that needs to be kept up to date as new builtin or 
target-specific fixups are added, We can calculate an appropriate bound on the 
number of bytes that might be touched using Info.TargetSize and 
Info.TargetOffset.

Differential Revision: https://reviews.llvm.org/D46965

llvm-svn: 333076