[ELF] Allow linker script to define symbol aliases

Fixes #610

elf/linker-script.cc

@@ -225,6 +225,10 @@ void parse_linker_script(Context<E> &ctx, MappedFile<Context<E>> *mf) {
       tok = skip(ctx, tok, "{");
       read_version_script(ctx, tok);
       tok = skip(ctx, tok, "}");
+    } else if (tok.size() > 3 && tok[1] == "=" && tok[3] == ";") {
+      ctx.arg.defsyms.emplace_back(get_symbol(ctx, unquote(tok[0])),
+                                   get_symbol(ctx, unquote(tok[2])));
+      tok = tok.subspan(4);
     } else if (tok[0] == ";") {
       tok = tok.subspan(1);
     } else {

test/elf/linker-script-defsym.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+export LC_ALL=C
+set -e
+CC="${TEST_CC:-cc}"
+CXX="${TEST_CXX:-c++}"
+GCC="${TEST_GCC:-gcc}"
+GXX="${TEST_GXX:-g++}"
+OBJDUMP="${OBJDUMP:-objdump}"
+MACHINE="${MACHINE:-$(uname -m)}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+t=out/test/elf/$MACHINE/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+void foo() {}
+EOF
+
+cat <<EOF > $t/script
+bar = foo;
+EOF
+
+$CC -B. -o $t/b.so -shared $t/script $t/a.o
+
+echo OK