Check for sysconf decl on vxworks

Check for sysconf decl on vxworks

The sysconf function is only available in rtp mode on vxworks.  In
kernel mode, it is not even declared, but the feature test macro in
the testsuite doesn't notice its absence because it's a link test, and
vxworks kernel mode uses partial linking.

This patch introduces an alternate test on vxworks targets to check
for a declaration and for an often-used sysconf parameter.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_sysconf):
	Check for declaration and _SC_PAGESIZE on vxworks.