[JITLink][RISCV] Consider relaxable call edges for PLT edges

For linker relaxation (D149526), a new edge kind (`CallRelaxable`) was
introduced. However, this new kind was not taken into account by
`PerGraphGOTAndPLTStubsBuilder_ELF_riscv`. This patch fixes this.

Reviewed By: StephenFan

Differential Revision: https://reviews.llvm.org/D150957