[RISCV][MC] Don't fold symbol differences if requiresDiffExpressionRelocations is true

When emitting the difference between two symbols, the standard behavior is 
that the difference will be resolved to an absolute value if both of the 
symbols are offsets from the same data fragment. This is undesirable on 
architectures such as RISC-V where relaxation in the linker may cause the 
computed difference to become invalid. This caused an issue when compiling to 
object code, where the size of a function in the debug information was already 
calculated even though it could change as a consequence of relaxation in the 
subsequent linking stage.

This patch inhibits the resolution of symbol differences to absolute values 
where the target's AsmBackend has declared that it does not want these to be 
folded.

Differential Revision: https://reviews.llvm.org/D45773
Patch by Edward Jones.

llvm-svn: 339864