x86/its: Move ITS indirect branch thunks to .text..__x86.indirect_thunk

The ITS mitigation includes both indirect branch thunks and return
thunks.  Both are currently placed in .text..__x86.return_thunk, which is
appropriate for the latter but not the former.

For consistency with other mitigations, move the indirect branch thunks to
.text..__x86.indirect_thunk.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>