uclibc: simplify UCLIBC_EXTRA_CFLAGS

Since commit f6695219 ("package/gcc:
remove gcc 4.8"), UCLIBC_EXTRA_CFLAGS no longer exists, and is
therefore always empty.

Passing the Buildroot make variable UCLIBC_EXTRA_CFLAGS into the
UCLIBC_EXTRA_CFLAGS fed into uClibc's make environment therefore no
longer makes sense. Therefore, just pass $(TARGET_ABI).

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