selftests/nolibc: libc-test: use HOSTCC instead of CC

libc-test is mainly added to compare the behavior of nolibc to the
system libc, it is meaningless and error-prone with cross compiling.

Let's use HOSTCC instead of CC to avoid wrongly use cross compiler when
CROSS_COMPILE is passed or customized.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Fixes: cfb672f9 ("selftests/nolibc: add run-libc-test target")
Signed-off-by: Willy Tarreau <w@1wt.eu>