Fix function pointer equality

Previously, a GOT relocation (e.g. R_X86_64_REX_GOTPCRELX) and a
R_X86_64_64 relocation referring the same imported symbols were
resolved to different addresses. Here is why:

 - When we saw a R_X86_64_64 relocation against an imported symbol,
   we created a PLT and resolve the relocation there.

 - GOT relocation is resolved to a GOT entry, which has a true
   address of an imported function at runtime, which is different
   from PLT entries that redirect calls to the real function.

With this patch, we no longer create a PLT entry for R_X86_64_64.
Instead, we emit a dynamic relocation so that it is always resolved
to a real function address.

Fixes GNU MP's `make check` failure, which was reported at
#81

arch_aarch64.cc

@@ -294,7 +294,7 @@ void InputSection<AARCH64>::scan_relocations(Context<AARCH64> &ctx) {
         // Absolute  Local    Imported data  Imported code
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // DSO
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // PIE
-        {  NONE,     NONE,    DYNREL,        PLT    },     // PDE
+        {  NONE,     NONE,    DYNREL,        DYNREL },     // PDE
       };
       dispatch(ctx, table, i);
       break;

arch_i386.cc

@@ -349,7 +349,7 @@ void InputSection<I386>::scan_relocations(Context<I386> &ctx) {
         // Absolute  Local    Imported data  Imported code
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // DSO
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // PIE
-        {  NONE,     NONE,    DYNREL,        PLT    },     // PDE
+        {  NONE,     NONE,    DYNREL,        DYNREL },     // PDE
       };
       dispatch(ctx, table, i);
       break;

arch_x86_64.cc

@@ -535,7 +535,7 @@ void InputSection<X86_64>::scan_relocations(Context<X86_64> &ctx) {
         // Absolute  Local    Imported data  Imported code
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // DSO
         {  NONE,     BASEREL, DYNREL,        DYNREL },     // PIE
-        {  NONE,     NONE,    DYNREL,        PLT    },     // PDE
+        {  NONE,     NONE,    DYNREL,        DYNREL },     // PDE
       };
       dispatch(ctx, table, i);
       break;

test/func-addr.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -e
+cd $(dirname $0)
+echo -n "Testing $(basename -s .sh $0) ... "
+t=$(pwd)/tmp/$(basename -s .sh $0)
+mkdir -p $t
+
+cat <<EOF | cc -shared -o $t/a.so -xc -
+void fn() {}
+EOF
+
+cat <<EOF | cc -o $t/b.o -c -xc -
+#include <stdio.h>
+
+void fn();
+void (*ptr)() = fn;
+
+int main() {
+  printf("%d\n", fn == ptr);
+}
+EOF
+
+clang -fuse-ld=`pwd`/../mold -o $t/exe $t/b.o $t/a.so
+$t/exe | grep -q 1
+
+echo OK