malloc: Remove memusage.h

And use machine-sp.h instead.  The Linux implementation is based on
already provided CURRENT_STACK_FRAME (used on nptl code) and
STACK_GROWS_UPWARD is replaced with _STACK_GROWS_UP.

sysdeps/unix/sysv/linux/machine-sp.h

@@ -0,0 +1,29 @@
+/* Machine-specific function to return the stack pointer.  Linux version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _MACHINE_SP_H
+#define _MACHINE_SP_H
+
+/* Return the current stack pointer.  */
+static inline uintptr_t
+__thread_stack_pointer (void)
+{
+  return (uintptr_t) CURRENT_STACK_FRAME;
+}
+
+#endif	/* machine-sp.h */