ObjectFile: add a case for EM_RISCV

This adds the jump slot mapping for RISCV.  This enables lldb to attach to a
remote debug server.  Although this doesn't enable debugging RISCV targets, it
is sufficient to attach, which is a slight improvement.

Tested with DebugServer2:
~~~
(lldb) gdb-remote localhost:1234
(lldb) Process 71438 stopped
* thread #1, name = 'reduced', stop reason = signal SIGTRAP
    frame #0: 0x0000003ff7fe1b20

error: Process 71438 is currently being debugged, kill the process before connecting.
(lldb) register read
general:
        x0 = 0x0000003ff7fe1b20
        x1 = 0x0000002ae00d3a50
        x2 = 0x0000003ffffff3e0
        x3 = 0x0000002ae01566e0
        x4 = 0x0000003fe567c7b0
        x5 = 0x0000000000001000
        x6 = 0x0000002ae00604ec
        x7 = 0x00000000000003ff
        x8 = 0x0000003fffc22db0
        x9 = 0x0000000000000000
       x10 = 0x0000000000000000
       x11 = 0x0000002ae603b1c0
       x12 = 0x0000002ae6039350
       x13 = 0x0000000000000000
       x14 = 0x0000002ae6039350
       x15 = 0x0000002ae6039350
       x16 = 0x73642f74756f3d5f
       x17 = 0x00000000000000dd
       x18 = 0x0000002ae6038f08
       x19 = 0x0000002ae603b1c0
       x20 = 0x0000002b0f3d3f40
       x21 = 0x0000003ff0b212d0
       x22 = 0x0000002b0f3a2740
       x23 = 0x0000002b0f3de3a0
       x24 = 0x0000002b0f3d3f40
       x25 = 0x0000002ad6929850
       x26 = 0x0000000000000000
       x27 = 0x0000002ad69297c0
       x28 = 0x0000003fe578b364
       x29 = 0x000000000000002f
       x30 = 0x0000000000000000
       x31 = 0x0000002ae602401a
        pc = 0x0000003ff7fe1b20
       ft0 = 0
       ft1 = 0
       ft2 = 0
       ft3 = 0
       ft4 = 0
       ft5 = 0
       ft6 = 0
       ft7 = 0
       fs0 = 0
       fs1 = 0
       fa0 = 0
       fa1 = 0
       fa2 = 0
       fa3 = 0
       fa4 = 0
       fa5 = 0
       fa6 = 0
       fa7 = 9.10304232197721e-313
       fs2 = 0
       fs3 = 1.35805727667792e-312
       fs4 = 1.35589259164679e-312
       fs5 = 1.35805727659887e-312
       fs6 = 9.10304232355822e-313
       fs7 = 0
       fs8 = 9.10304233027751e-313
       fs9 = 0
      fs10 = 9.10304232948701e-313
      fs11 = 1.35588724164707e-312
       ft8 = 0
       ft9 = 9.1372158616833e-313
      ft10 = 9.13720376537528e-313
      ft11 = 1.356808717416e-312
3 registers were unavailable.

(lldb) disassemble
error: Failed to disassemble memory at 0x3ff7fe1b2
~~~

Author: compnerd

lldb/source/Plugins/ObjectFile/ELF/ELFHeader.cpp

@@ -208,6 +208,9 @@ unsigned ELFHeader::GetRelocationJumpSlotType() const {
   case EM_S390:
     slot = R_390_JMP_SLOT;
     break;
+  case EM_RISCV:
+    slot = R_RISCV_JUMP_SLOT;
+    break;
   }
 
   return slot;