xtensa: Avoid designated inits, for C++ compliance

xtensa: Avoid designated inits, for C++ compliance

C++ does not officially support designators in initializer lists.  Thus
some compilers may issue errors when encountering them.  Modern versions
of GCC seem to allow them by default, as a GCC extension, even though
the GCC documentation explicitly states otherwise: "[...] This extension
is not implemented in GNU C++."  But some older GCC versions (like
4.4.7) did indeed emit an error instead, like this:

  .../gdb/xtensa-config.c:219: error: expected primary-expression before
			       ‘.’ token

This patch removes the only such instance I've seen when building with
'--enable-targets=all'.

gdb/ChangeLog:

	* xtensa-tdep.h (XTENSA_GDBARCH_TDEP_INSTANTIATE): Replace
	designated initializer list by plain initializer list, for C++
	compliance.