[RISCV][MC] Add support for evaluating constant symbols as immediates

This further improves compatibility with GNU as, allowing input such as the
following to be assembled:

.equ CONST, 0x123456
li a0, CONST
addi a0, a0, %lo(CONST)

.equ CONST, 1
slli a0, a0, CONST

Note that we don't have perfect compatibility with gas, as it will avoid
emitting a relocation in this case:

addi a0, a0, %lo(CONST2)
.equ CONST2, 0x123456

Thanks to Shiva Chen for suggesting a better way to approach this during review.

Differential Revision: https://reviews.llvm.org/D52298

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350831 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed