Add noexcept to custom non-throwing new operators.

Both libc++ and libstdc++ declare non-throwing new operators as
noexcept and overloads must also be noexcept.  This fixes a
-Wmissing-exception-spec warning with clang.

gdb/ChangeLog:

	* common/new-op.c (operator new): Mark 'noexcept'.
	(operator new[]): Likewise.

gdb/common/new-op.c

@@ -76,7 +76,7 @@ operator new (std::size_t sz)
 }
 
 void *
-operator new (std::size_t sz, const std::nothrow_t&)
+operator new (std::size_t sz, const std::nothrow_t&) noexcept
 {
   /* malloc (0) is unpredictable; avoid it.  */
   if (sz == 0)
@@ -91,7 +91,7 @@ operator new[] (std::size_t sz)
 }
 
 void*
-operator new[] (std::size_t sz, const std::nothrow_t&)
+operator new[] (std::size_t sz, const std::nothrow_t&) noexcept
 {
   return ::operator new (sz, std::nothrow);
 }