[gdb/testsuite] Don't use O2 for inlining in break-inline-psymtab.exp

[gdb/testsuite] Don't use O2 for inlining in break-inline-psymtab.exp

In test-case gdb.dwarf2/break-inline-psymtab.exp we use O2 to enable inlining
of bar into foo in break-inline-psymtab-2.c.

Instead, enforce inlining using __attribute__((always_inline)), to avoid any
optimization-related test issues.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-02  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/break-inline-psymtab-2.c (bar): Add
	__attribute__((always_inline)).
	* gdb.dwarf2/break-inline-psymtab.exp: Don't use -O2.