[ELF] Allow relative relocs against absolute symbols

In the previous commit (c325cc0),
I argued that PC-relative relocations are not representable if they
refer absolute symbols, and I made a change so that such relocations
are handled as errors.

Even though what I did is technically correct, that results in a failure
of an important use case of the weak undefined symbol. Here is why.

If you have an weak undefined symbol `foo`, you would use it as follows:

  if (foo)
    foo();

If `foo` is defined (i.e. has an address other than 0), `foo` is called.

If `foo` is undefined (i.e. has the address 0), the subsequent CALL
instruction will have an offset from the CALL instruction to address 0.
That displacement is computed at link-time. If the output is position-
independent, the call instruction may not have a correct displacement
from address 0 to the instruction at runtime due to base relocation.

However, that's not a problem in practice, because the function call
is guarded by `if (foo)`, and that will always evaluated to false
if `foo` is undefined. Therefore, the faulty `CALL` instruction will
never be executed.

So, that means we need to relocate PC-relative relocations against
absolute symbols even if doing so results in an incorrect result.
This patch implement that.

elf/arch-i386.cc

@@ -425,8 +425,8 @@ void InputSection<E>::scan_relocations(Context<E> &ctx) {
     case R_386_PLT32: {
       Action table[][4] = {
         // Absolute  Local  Imported data  Imported code
-        {  ERROR,    NONE,  PLT,           PLT    },     // DSO
-        {  ERROR,    NONE,  PLT,           PLT    },     // PIE
+        {  NONE,     NONE,  PLT,           PLT    },     // DSO
+        {  NONE,     NONE,  PLT,           PLT    },     // PIE
         {  NONE,     NONE,  PLT,           PLT    },     // PDE
       };
       dispatch(ctx, table, i, rel, sym);

elf/arch-x86-64.cc

@@ -668,8 +668,8 @@ void InputSection<E>::scan_relocations(Context<E> &ctx) {
     case R_X86_64_PLTOFF64: {
       Action table[][4] = {
         // Absolute  Local  Imported data  Imported code
-        {  ERROR,    NONE,  PLT,           PLT    },     // DSO
-        {  ERROR,    NONE,  PLT,           PLT    },     // PIE
+        {  NONE,     NONE,  PLT,           PLT    },     // DSO
+        {  NONE,     NONE,  PLT,           PLT    },     // PIE
         {  NONE,     NONE,  PLT,           PLT    },     // PDE
       };
       dispatch(ctx, table, i, rel, sym);

test/elf/weak-undef.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+export LANG=
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t=out/test/elf/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -c -o $t/a.o -fPIC -xc -
+#include <stdio.h>
+__attribute__((weak)) int foo();
+int main() {
+  printf("%d\n", foo ? foo() : -1);
+}
+EOF
+
+cat <<EOF | $CC -c -o $t/b.o -fno-PIC -xc -
+#include <stdio.h>
+__attribute__((weak)) int foo();
+int main() {
+  printf("%d\n", foo ? foo() : -1);
+}
+EOF
+
+cat <<EOF | $CC -fcommon -xc -c -o $t/c.o -
+int foo() { return 2; }
+EOF
+
+$CC -B. -o $t/exe1 $t/a.o -pie
+$CC -B. -o $t/exe2 $t/b.o -no-pie
+$CC -B. -o $t/exe3 $t/a.o $t/c.o -pie
+$CC -B. -o $t/exe4 $t/b.o $t/c.o -no-pie
+
+$t/exe1 | grep -q '^-1$'
+$t/exe2 | grep -q '^-1$'
+$t/exe3 | grep -q '^2$'
+$t/exe4 | grep -q '^2$'
+
+echo OK