add caching procs to test suite

add caching procs to test suite

In the fully parallel mode, each .exp file can be run in parallel (at
least conceptually -- the actual split may not be so severe).  This
means that procs that compute a result and cache it are not going to
function very well.  The test they run will be invoked over and over.

This patch introduces a generic caching mechanism and changes various
result-caching procs to use it.  This is a cleanup to introduce the
basic change; the results aren't written to disk yet.

A caching proc is defined using gdb_caching_proc, which works like
"proc", except that it caches the result of the body.

	* lib/cache.exp: New file.
	* lib/cell.exp (skip_cell_tests): Use gdb_caching_proc.
	* lib/gdb.exp: Load cache.exp.
	(support_complex_tests, is_ilp32_target, is_lp64_target)
	(is_amd64_regs_target, skip_altivec_tests, skip_vsx_tests)
	(gdb_skip_xml_test): Use gdb_caching_proc.
	* lib/opencl.exp (skip_opencl_tests): Use gdb_caching_proc.