libc/glibc: add extra CFLAGS when building the C library

When building multilib, we need extra CFLAGS that tell the compiler
to use non-default settings (eg. big/little endian, hard/soft float,
-march/cpu/tune flags, and so on...).

We have to pass these flags to the build.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Giammarco Zacheo"

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