macaw-riscv: Add PLTStubInfo for RISC-V programs

[RyanGlScott]

Currently, we do not provide a PLTStubInfo value for RISC-V in macaw-riscv. I think it would be relatively straightforward to define one, however. Using binaries produced by riscv{32,64}-linux-musl-gcc as a baseline, here is a first attempt:

riscvPLTStubInfo = PLTStubInfo
  { pltFunSize     = 32
  , pltStubSize    = 16
  , pltGotStubSize = error "Unexpected .plt.got section in RISC-V binary"
  }

This is probably best done in conjunction with GaloisInc/elf-edit#36.

After we add riscvPLTStubInfo, make sure to use it here (and add a test case demonstrating that it works):

macaw/macaw-riscv-symbolic/tests/Main.hs

Lines 150 to 155 in 1a2b928

	binfo <- MST.runDiscovery ehi exePath MST.toAddrSymMap archInfo
	-- Test cases involving shared libraries are not
	-- yet supported on the RISC-V backend. At a
	-- minimum, this is blocked on
	-- https://github.com/GaloisInc/elf-edit/issues/36.
	(MMELP.noPLTStubInfo "RISC-V")