gawk: remove CC override

For some unknown reason, gawk.mk was overriding CC to $(TARGET_CC) at
build time, while it had already been set at configure time to
"$(TARGET_CC) $(TARGET_CFLAGS)" which is already the good value.

Setting it to just $(TARGET_CC) breaks the compilation with external
toolchains because we must pass the --sysroot option. But anyway, this
was incorrect as we were loosing some CFLAGS set by Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>