[gdb/testsuite] Compile infcall-nested-structs.exp with -O2

[gdb/testsuite] Compile infcall-nested-structs.exp with -O2

As mentioned in commit 745ff14e6e1 "[gdb/tdep] Fix 'Unexpected register class'
assert in amd64_push_arguments", of the 12 KFAILs added there, 3 are KPASSing
with g++ 4.8.5.

The KPASSes are due to:
- gdb incorrectly expecting the second half of the result of function
  rtn_str_struct_02_01 in register %rdx.
- rtn_str_struct_02_01 using %rdx as a temporary, thereby accidentally setting
  it to the expected value.

Reduce the chance of hiding errors due accidental register settings by
compiling the test-case with -O2.

This fixes the KPASSes when applied on top of commit 745ff14e6e1.

Tested on x86_64-linux.

Tested with g++ 4.8.5, 7.4.1, 8.3.1, 9.2.1.

gdb/testsuite/ChangeLog:

2019-10-21  Tom de Vries  <tdevries@suse.de>

	* gdb.base/infcall-nested-structs.c: Add
	__attribute__((noinline,noclone)) to all functions.
	(call_all): Add missing variable initialization.  Simplify return value.
	(breakpt): Increment volatile variable, to prevent call from being
	optimized out.
	* gdb.base/infcall-nested-structs.exp: Compile with -O2.

Change-Id: Ic027e1c957fecd6686345639db99f5eaee3cdf05