[JITLink][x86-64] Implement ELF::R_X86_64_16.

We already had an x86_64::Pointer16 edge kind courtesy of Sunho's COFF/x86-64
work, it just needed to be wired up.

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

@@ -159,6 +159,9 @@ class ELFLinkGraphBuilder_x86_64 : public ELFLinkGraphBuilder<object::ELF64LE> {
     case ELF::R_X86_64_32:
       Kind = x86_64::Pointer32;
       break;
+    case ELF::R_X86_64_16:
+      Kind = x86_64::Pointer16;
+      break;
     case ELF::R_X86_64_32S:
       Kind = x86_64::Pointer32Signed;
       break;

llvm/test/ExecutionEngine/JITLink/x86-64/ELF_R_X86_64_16.s

@@ -0,0 +1,33 @@
+# RUN: llvm-mc -triple=x86_64-unknown-linux -position-independent \
+# RUN:     -filetype=obj -o %t.o %s
+# RUN: llvm-jitlink -noexec -abs X=0x1234 -check=%s %t.o
+# RUN: not llvm-jitlink -noexec -abs X=0x12345 %t.o 2>&1 | \
+# RUN:   FileCheck -check-prefix=CHECK-ERROR %s
+#
+# Check success and failure cases of R_X86_64_16 handling.
+
+# jitlink-check: *{8}P = X
+
+# CHECK-ERROR: relocation target "X" {{.*}} is out of range of Pointer16 fixup
+
+	.text
+	.section	.text.main,"ax",@progbits
+	.globl	main
+	.p2align	4, 0x90
+	.type	main,@function
+main:
+	xorl	%eax, %eax
+	retq
+.Lfunc_end0:
+	.size	main, .Lfunc_end0-main
+
+	.type	P,@object
+	.data
+	.globl	P
+	.p2align	1, 0x0
+P:
+	.short	X    # Using short here generates R_X86_64_16.
+	.short  0
+	.short  0
+	.short  0
+	.size	P, 8