guix: Remove now-unnecessary gcc make flag
guix: Remove now-unnecessary gcc make flag

Previously, Guix would produce a gcc which did not know to use the SSP
function from glibc, and required a gcc make flag for it to do so, in my
attempt to fix it upstream I realized that this is no longer the case.

This can be verified by performing a Guix build and doing

  readelf -s ... | grep __stack_chk

to check that symbols are coming from glibc, and doing

  readelf -d ... | grep NEEDED | grep ssp

to see that libssp.so is not being depended on

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