run sysconf-requiring test on systems that support it

Some gcc.target/i386 tests requires the mmap feature, but that's not
enough for the test to be able to call sysconf.

This patch introduces a sysconf feature, analogous to mmap, and adds
it to tests in gcc.target/i386 that call sysconf.

There are other tests within gcc.dg and g++.dg that call sysconf, but
I haven't added the tag to them, because they already cover it with
target triplets.  I was a little nervous about dropping the triplets,
and saw how they implied sysconf, so I left those alone.


for  gcc/ChangeLog

	* doc/sourcebuild.texi (sysconf): New effective target.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_sysconf): New.
	* gcc.target/i386/pr95443-1.c: Require it.
	* gcc.target/i386/pr95443-2.c: Likewise.
	* gcc.target/i386/sse2-mmx-maskmovq.c: Likewise.
	* gcc.target/i386/strncmp-1.c: Likewise.

gcc/doc/sourcebuild.texi

@@ -2450,6 +2450,9 @@ all targets.
 @item stpcpy
 Target provides @code{stpcpy} function.
 
+@item sysconf
+Target supports @code{sysconf}.
+
 @item trampolines
 Target supports trampolines.
 

gcc/testsuite/gcc.target/i386/pr95443-1.c

@@ -1,4 +1,4 @@
-/* { dg-do run { target mmap } } */
+/* { dg-do run { target { sysconf && mmap } } } */
 /* { dg-options "-O2 -minline-all-stringops" } */
 
 #include <stdint.h>

gcc/testsuite/gcc.target/i386/pr95443-2.c

@@ -1,4 +1,4 @@
-/* { dg-do run { target mmap } } */
+/* { dg-do run { target { sysconf && mmap } } } */
 /* { dg-options "-O2 -minline-all-stringops" } */
 
 #include <stdint.h>

gcc/testsuite/gcc.target/i386/sse2-mmx-maskmovq.c

@@ -1,4 +1,4 @@
-/* { dg-do run { target mmap } } */
+/* { dg-do run { target { sysconf && mmap } } } */
 /* { dg-options "-O2 -fno-strict-aliasing -msse2" } */
 /* { dg-additional-options "-mno-mmx" { target { ! ia32 } } } */
 

gcc/testsuite/gcc.target/i386/strncmp-1.c

@@ -1,4 +1,4 @@
-/* { dg-do run { target mmap } } */
+/* { dg-do run { target { sysconf && mmap } } } */
 /* { dg-options "-O2" } */
 
 #include <stdio.h>

gcc/testsuite/lib/target-supports.exp

@@ -1067,6 +1067,12 @@ proc check_effective_target_mmap {} {
     return [check_function_available "mmap"]
 }
 
+# Return 1 if the target supports sysconf, 0 otherwise.
+
+proc check_effective_target_sysconf {} {
+    return [check_function_available "sysconf"]
+}
+
 # Return 1 if the target supports dlopen, 0 otherwise.
 proc check_effective_target_dlopen {} {
     return [check_no_compiler_messages dlopen executable {