gcc-target: Do not set --with-sysroot and gxx-include-dir paths

gcc-target: Do not set --with-sysroot and gxx-include-dir paths

These options are not needed on target infact since the defaults would
be good enough for compiler to find the relevant headers and libraries
from compiler runtime

with gcc9 it starts to strip the sysroot from gxx-include-dir which
means it tries to look for gxx headers in localdir

ignoring nonexistent directory "usr/include/c++/9.0.1"
ignoring nonexistent directory "usr/include/c++/9.0.1/arm-yoe-linux-gnueabi"
ignoring nonexistent directory "usr/include/c++/9.0.1/backward"

instead of sysroot

Removing these options make it behave normal

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>