x86: re-enable OPTIMIZE_INLINING

re-enable OPTIMIZE_INLINING more widely. Jeff Dike fixed the remaining
outstanding issue in this commit:

| commit 4f81c535
| Author: Jeff Dike <jdike@addtoit.com>
| Date:   Mon Jul 7 13:36:56 2008 -0400
|
|     [UML] fix gcc ICEs and unresolved externs
[...]
|    This patch reintroduces unit-at-a-time for gcc >= 4.0, bringing back the
|    possibility of Uli's crash.  If that happens, we'll debug it.

it's still default-off and thus opt-in.

Signed-off-by: Ingo Molnar <mingo@elte.hu>