consistently use empty toolchain version in ifort easyconfigs to ensure that GCC(core) dep is loaded during installation

[boegel]

The GCCcore module on top of which ifort is often installed is not loaded when 'dummy' is used as version, which affects the outcome of gcc -print-multiarch, see easybuilders/easybuild-easyblocks#1237 and easybuilders/easybuild-easyblocks#1249.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 31 out of 31 (31 easyconfigs in this PR)
node2700.swalot.os - Linux centos linux 7.3.1611, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz, Python 2.7.5
See https://gist.github.com/8e61905728dc58a47852bda8c927523c for a full test report.

[boegel]

Test report by @boegel
FAILED
Build succeeded for 29 out of 31 (31 easyconfigs in this PR)
node2011.delcatty.os - Linux centos linux 7.3.1611, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.7.5
See https://gist.github.com/515a4a23ab73c85a33f5b0627c08690d for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 31 out of 31 (31 easyconfigs in this PR)
node2113.delcatty.os - Linux centos linux 7.3.1611, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.7.5
See https://gist.github.com/e407b6901aaa2beddf089b172675d437 for a full test report.