xtensa: initialize call_abi in xtensa_tdep

xtensa: initialize call_abi in xtensa_tdep

Use XSHAL_ABI value provided by xtensa-config.h to correctly initialize
xtensa_tdep.call_abi
This fixes calls to functions from GDB that otherwise fail with the
following assertion in call0 configuration:

  gdb/regcache.c:602: internal-error: regcache_raw_read: Assertion
  `regnum >= 0 && regnum < regcache->descr->nr_raw_registers' failed.

2015-08-31  Max Filippov  <jcmvbkbc@gmail.com>
gdb/
	* xtensa-tdep.h (XTENSA_GDBARCH_TDEP_INSTANTIATE): Initialize
	call_abi using XSHAL_ABI macro.