busybox: Skip a non-posix awk ptest on musl systems

busybox: Skip a non-posix awk ptest on musl systems

The test depends on BSD extension REG_STARTEND which
is not in POSIX, glibc does implement this extention so it
works on glibc systems but musl does not and it needs to
be skipped and LIBC environment variable is used in this
test as well to run it conditionally.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>