Require GCC 5 or above for 3 x86 tests which fail with GCC 4.9.3.
PR ld/21924
* testsuite/ld-i386/i386.exp: Require GCC 5 or above for
"weakundef1 with PIE" test.
* testsuite/ld-x86-64/tls.exp: Require GCC 5 or above for
"tlsdesc1" and "tlsdesc1 with PIE" tests.