or1k: Only define TARGET_HAVE_TLS when HAVE_AS_TLS

or1k: Only define TARGET_HAVE_TLS when HAVE_AS_TLS

This was found when testing buildroot with linuxthreads enabled.  In
this case, the build passes --disable-tls to the toolchain during
configuration.  After building the OpenRISC toolchain it was still
generating TLS code sequences and causing linker failures such as:

 ..../or1k-buildroot-linux-uclibc-gcc -o gpsd-3.24/gpsctl .... -lusb-1.0 -lm -lrt -lnsl
 ..../ld: ..../sysroot/usr/lib/libusb-1.0.so: undefined reference to `__tls_get_addr'

This patch fixes this by disabling tls for the OpenRISC target when requested
via --disable-tls.

gcc/ChangeLog:

	* config/or1k/or1k.cc (TARGET_HAVE_TLS): Only define if
	HAVE_AS_TLS is defined.

Tested-by: Yann E. MORIN <yann.morin@orange.com>