[ELF] Do not use std::has_single_bit

It looks like some stdlib implementations do not provide that function.

Fixes #308

elf/cmdline.cc

@@ -645,7 +645,7 @@ void parse_nonpositional_args(Context<E> &ctx,
       ctx.arg.z_execstack = true;
     } else if (read_z_arg(ctx, args, arg, "max-page-size")) {
       ctx.page_size = parse_number(ctx, "-z max-page-size", arg);
-      if (!std::has_single_bit<u64>(ctx.page_size))
+      if (std::popcount<u64>(ctx.page_size) != 1)
         Fatal(ctx) << "-z max-page-size " << arg << ": value must be a power of 2";
     } else if (read_z_flag(args, "noexecstack")) {
       ctx.arg.z_execstack = false;

mold.h

@@ -161,12 +161,12 @@ class Warn {
 inline u64 align_to(u64 val, u64 align) {
   if (align == 0)
     return val;
-  assert(std::has_single_bit(align));
+  assert(std::popcount(align) == 1);
   return (val + align - 1) & ~(align - 1);
 }
 
 inline u64 align_down(u64 val, u64 align) {
-  assert(std::has_single_bit(align));
+  assert(std::popcount(align) == 1);
   return val & ~(align - 1);
 }
 
@@ -309,7 +309,7 @@ class ConcurrentMap {
     if (!keys)
       return {nullptr, false};
 
-    assert(std::has_single_bit<u64>(nbuckets));
+    assert(std::popcount<u64>(nbuckets) == 1);
     i64 idx = hash & (nbuckets - 1);
     i64 retry = 0;