test-cases: fix incorrectly linking libc when backend is llvm

Now link_libc=1 must be used to link with libc, instead of the test
harness assuming that using the llvm backend means additionally linking
with libc.

Author: andrewrk

test/cases/f32_passed_to_variadic_fn.zig

@@ -9,7 +9,8 @@ pub fn main() void {
 // run
 // backend=llvm
 // target=x86_64-linux-gnu
+// link_libc=1
 //
 // f64: 2.000000
 // f32: 10.000000
-//
+//

test/cases/fn_typeinfo_passed_to_comptime_fn.zig

@@ -14,4 +14,5 @@ fn foo(comptime info: std.builtin.Type) !void {
 
 // run
 // is_test=1
+// backend=llvm
 //

test/cases/llvm/hello_world.zig

@@ -7,6 +7,7 @@ pub fn main() void {
 // run
 // backend=llvm
 // target=x86_64-linux,x86_64-macos
+// link_libc=1
 //
 // hello world!
 //

test/src/Cases.zig

@@ -382,6 +382,7 @@ fn addFromDirInner(
         const backends = try manifest.getConfigForKeyAlloc(ctx.arena, "backend", Backend);
         const targets = try manifest.getConfigForKeyAlloc(ctx.arena, "target", CrossTarget);
         const is_test = try manifest.getConfigForKeyAssertSingle("is_test", bool);
+        const link_libc = try manifest.getConfigForKeyAssertSingle("link_libc", bool);
         const output_mode = try manifest.getConfigForKeyAssertSingle("output_mode", std.builtin.OutputMode);
 
         var cases = std.ArrayList(usize).init(ctx.arena);
@@ -397,7 +398,7 @@ fn addFromDirInner(
                     .updates = std.ArrayList(Cases.Update).init(ctx.cases.allocator),
                     .is_test = is_test,
                     .output_mode = output_mode,
-                    .link_libc = backend == .llvm,
+                    .link_libc = link_libc,
                     .deps = std.ArrayList(DepModule).init(ctx.cases.allocator),
                 });
                 try cases.append(next);
@@ -745,6 +746,8 @@ const TestManifestConfigDefaults = struct {
             };
         } else if (std.mem.eql(u8, key, "is_test")) {
             return "0";
+        } else if (std.mem.eql(u8, key, "link_libc")) {
+            return "0";
         } else unreachable;
     }
 };