coreutils: Delete gcc sysroot parameter for ptest on target

coreutils: Delete gcc sysroot parameter for ptest on target

If gcc is installed in image, ptest result has 4 ERROR.
  ERROR: tests/rm/r-root.sh
  ERROR: tests/rm/rm-readdir-fail.sh
  ERROR: tests/cp/nfs-removal-race.sh
  ERROR: tests/ls/getxattr-speedup.sh

r-root.log as an example:
  --------------------------
  k.c:1:10: fatal error: stdio.h: No such file or directory
  1 | #include <stdio.h>
    |  ^~~~~~~~~
  compilation terminated.
  r-root.sh: set-up failure: failed to build shared library
  ERROR tests/rm/r-root.sh (exit status: 99)
  --------------------------

reason:
  The run-ptest calls make cmd to run test cases.
  In these cases, k.c file is created and compiled by gcc before run.
  There is a stdio.h file in /usr/include/ directory.
  Normally, gcc has /usr/include as part of its default search path.
  But in Makefile, it has the "--sysroot=recipe-sysroot" parameter
  which makes it does not work.

solution:
  Delete "--sysroot=recipe-sysroot" from Makefile.

other:
  If gcc is not installed in image, these cases will be skipped.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>