libphobos: Fix FAIL phobos.exp/core.time on CentOS 5.11, Linux 2.6.18

libphobos: Fix FAIL phobos.exp/core.time on CentOS 5.11, Linux 2.6.18

Merges upstream druntime e03164b5.

Reviewed-on: https://github.com/dlang/druntime/pull/2581

libphobos/ChangeLog:

2019-04-24  Iain Buclaw  <ibuclaw@gdcproject.org>

	PR d/89432
	* testsuite/lib/libphobos.exp (check_effective_target_linux_pre_2639):
	New proc.
	* testsuite/libphobos.druntime/druntime.exp: Add compiler flag
	-fversion=Linux_Pre_2639 if target is linux_pre_2639.
	* testsuite/libphobos.druntime_shared/druntime_shared.exp: Likewise.

From-SVN: r270554