Reduce <limits.h> pollution due to dynamic PTHREAD_STACK_MIN

<limits.h> used to be a header file with no declarations.
GCC's libgomp includes it in a #pragma GCC visibility hidden block.
Including <unistd.h> from <limits.h> (indirectly) declares everything
in <unistd.h> with hidden visibility, resulting in linker failures.

This commit avoids C declarations in assembler mode and only declares
__sysconf in <limits.h> (and not the entire contents of <unistd.h>).
The __sysconf symbol is already part of the ABI.  PTHREAD_STACK_MIN
is no longer defined for __USE_DYNAMIC_STACK_SIZE && __ASSEMBLER__
because there is no possible definition.

Additionally, PTHREAD_STACK_MIN is now defined by <pthread.h> for
__USE_MISC because this is what developers expect based on the macro
name.  It also helps to avoid libgomp linker failures in GCC because
libgomp includes <pthread.h> before its visibility hacks.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

include/bits/local_lim.h

@@ -1,7 +0,0 @@
-/* Don't define PTHREAD_STACK_MIN to sysconf (_SC_THREAD_STACK_MIN) for
-   glibc build.  */
-#if !defined _ISOMAC
-# undef __USE_DYNAMIC_STACK_SIZE
-#endif
-
-#include_next <bits/local_lim.h>

nptl/tst-minstack-exit.c

@@ -24,6 +24,9 @@
 #include <support/check.h>
 #include <support/xthread.h>
 
+_Static_assert (__SC_THREAD_STACK_MIN_VALUE == _SC_THREAD_STACK_MIN,
+                "__SC_THREAD_STACK_MIN_VALUE is correct");
+
 static void *
 threadfunc (void *closure)
 {

sysdeps/nptl/pthread.h

@@ -29,7 +29,9 @@
 #include <bits/types/struct_timespec.h>
 #include <bits/types/__sigset_t.h>
 #include <bits/types/struct___jmp_buf_tag.h>
-
+#ifdef __USE_MISC
+# include <bits/pthread_stack_min-dynamic.h>
+#endif
 
 /* Detach state.  */
 enum

sysdeps/unix/sysv/linux/Makefile

@@ -109,7 +109,7 @@ sysdep_headers += sys/mount.h sys/acct.h \
 		  bits/types/struct_semid64_ds_helper.h \
 		  bits/types/struct_shmid64_ds.h \
 		  bits/types/struct_shmid64_ds_helper.h \
-		  bits/pthread_stack_min.h
+		  bits/pthread_stack_min.h bits/pthread_stack_min-dynamic.h
 
 tests += tst-clone tst-clone2 tst-clone3 tst-fanotify tst-personality \
 	 tst-quota tst-sync_file_range tst-sysconf-iov_max tst-ttyname \

sysdeps/unix/sysv/linux/bits/local_lim.h

@@ -77,14 +77,8 @@
    priority level.  */
 #define AIO_PRIO_DELTA_MAX	20
 
-/* Minimum size for a thread.  We are free to choose a reasonable value.  */
-#undef PTHREAD_STACK_MIN
-#if defined __USE_DYNAMIC_STACK_SIZE && __USE_DYNAMIC_STACK_SIZE
-# include <unistd.h>
-# define PTHREAD_STACK_MIN	sysconf (_SC_THREAD_STACK_MIN)
-#else
-# include <bits/pthread_stack_min.h>
-#endif
+/* Arrange for the definition of PTHREAD_STACK_MIN.  */
+#include <bits/pthread_stack_min-dynamic.h>
 
 /* Maximum number of timer expiration overruns.  */
 #define DELAYTIMER_MAX	2147483647

sysdeps/unix/sysv/linux/bits/pthread_stack_min-dynamic.h

@@ -0,0 +1,31 @@
+/* Definition of PTHREAD_STACK_MIN, possibly dynamic.
+   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 PTHREAD_STACK_MIN
+# if defined __USE_DYNAMIC_STACK_SIZE && __USE_DYNAMIC_STACK_SIZE
+#  ifndef __ASSEMBLER__
+#   define __SC_THREAD_STACK_MIN_VALUE 75
+__BEGIN_DECLS
+extern long int __sysconf (int __name) __THROW;
+__END_DECLS
+#   define PTHREAD_STACK_MIN __sysconf (__SC_THREAD_STACK_MIN_VALUE)
+#  endif
+# else
+#  include <bits/pthread_stack_min.h>
+# endif
+#endif

sysdeps/unix/sysv/linux/include/bits/pthread_stack_min-dynamic.h

@@ -0,0 +1,7 @@
+/* Don't define PTHREAD_STACK_MIN to __sysconf (_SC_THREAD_STACK_MIN)
+   for glibc build.  */
+#ifdef _ISOMAC
+# include <sysdeps/unix/sysv/linux/bits/pthread_stack_min-dynamic.h>
+#else
+# include <bits/pthread_stack_min.h>
+#endif