[3.12] gh-105776: Fix test_cppext when CC contains -std=c11 option (GH-108343) (#108345)

gh-105776: Fix test_cppext when CC contains -std=c11 option (GH-108343)

Fix test_cppext when the C compiler command has the "-std=c11" option.
Remove "-std=" options from the compiler command.
(cherry picked from commit 9173b2bbe13aeccc075b571da05c653a2a91de1b)

Co-authored-by: Victor Stinner <vstinner@python.org>