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>

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