Configure: Fix check for --target support in stage0 CC

Andreas Klebingerrequested to merge
wip/andreask/config-linker-target into master 
Configure: Fix check for --target support in stage0 CC

The check FP_PROG_CC_LINKER_TARGET used $CC unconditionally to check for
--target support. However this fails for the stage0 config where the C
compiler used is not $CC but $CC_STAGE0.

Since we already pass the compiler under test into the macro I simply
changed it to use that instead.

Fixes #26999

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