scripts/gdb: If not building expat for gdb, disable

--with-expat=yes is unconditionally passed to the gdb configure
stage, instead of respecting the ${do_expat} decision.

Disable if not needed. Prevents error building canadian cross;

configure: error: expat is missing or unusable

Where configure stage fails to find expat on the host compiler.

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Message-Id: <4c4410a2a8aab24a29c5.1349244128@localhost.localdomain>
PatchWork-Id: 188711

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_BE=y

 CT_ARCH_FLOAT_SW=y

 CT_ARCH_arm=y

 CT_BINUTILS_V_2_19_1a=y

 CT_CC_V_4_3_2=y

 CT_LIBC_none=y

 CT_DEBUG_gdb=y

 CT_GMP_V_4_3_2=y

 CT_MPFR_V_2_4_2=y