Initialize target description early in IPA

Target descriptions are allocated lazily, that is fine in GDBserver,
but it is not safe to call malloc in gdb_collect in IPA, because we
can set a fast tracepoint in malloc, and when the tracepoint is hit,
gdb_collect/malloc is called, deadlock or memory corruption may be
triggered.

 #0  0xf7cfc200 in malloc ()
 #1  0xf7efdc07 in operator new(unsigned int) ()
 #2  0xf7ef7636 in allocate_target_description() ()
 #3  0xf7efcbe1 in i386_create_target_description(unsigned long long, bool) ()
 #4  0xf7efb474 in i386_linux_read_description(unsigned long long) ()
 #5  0xf7efb190 in get_ipa_tdesc(int) ()
 #6  0xf7ef9baa in gdb_collect ()

The fix is to initialize all target descriptions earlier, when the
IPA is loaded.  In order to guarantee malloc is not called in IPA
in gdb_collect, I change the test to set a breakpoint on malloc, if
IPA gdb_collect calls malloc, program will hit the breakpoint, and
test fail.

continue
Continuing.

Thread 1 "" hit Breakpoint 5, 0xf7cfc200 in malloc ()
(gdb) FAIL: gdb.trace/ftrace.exp: advance through tracing

gdb/gdbserver:

2017-12-07  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-ipa.c (initialize_low_tracepoint): Call
	aarch64_linux_read_description.
	* linux-amd64-ipa.c (idx2mask): New array.
	(get_ipa_tdesc): Move idx2mask out.
	(initialize_low_tracepoint): Initialize target descriptions.
	* linux-i386-ipa.c (idx2mask): New array.
	(get_ipa_tdesc): Move idx2mask out.
	(initialize_low_tracepoint): Initialize target descriptions.

gdb/testsuite:

2017-12-07  Yao Qi  <yao.qi@linaro.org>

	* gdb.trace/ftrace.exp (run_trace_experiment): Set breakpoint on
	malloc and catch syscall.