include/trusted-firmware-a.mk: unset CC before compiling

include/trusted-firmware-a.mk: unset CC before compiling

If CC is explicitly set to a non default value, TF-A keeps this value.
Otherwise it assigns generic default value.
As the build bot set CC=/usr/bin/gcc-10, TF-A uses it which causes a
compile issue.
So unset CC before compiling.

It's a new behavior since v2.12. A part of the explanation is available in
this commit [1].

[1] https://github.com/ARM-software/arm-trusted-firmware/commit/e01c71266f9df46ac46dc72669449490d1c94419

Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://github.com/openwrt/openwrt/pull/17465
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>