x86: mark read_crX() asm code as volatile

Some gcc versions (I checked at least 4.1.1 from RHEL5 & 4.1.2 from gentoo)
can generate incorrect code with read_crX()/write_crX() functions mix up,
due to cached results of read_crX().

The small app for x8664 below compiled with -O2 demonstrates this
(i686 does the same thing):

1 files changed, 1 insertions, 1 deletions

diff --git a/include/asm-x86/system_32.h b/include/asm-x86/system_32.h
index d84e593b7dfc..1d6fb3afa533 100644
--- a/include/asm-x86/system_32.h
+++ b/include/asm-x86/system_32.h
@@ -142,7 +142,7 @@ static inline unsigned long native_read_cr4_safe(void)
 {
         unsigned long val;
         /* This could fault if %cr4 does not exist */
-        asm("1: movl %%cr4, %0          \n"
+        asm volatile("1: movl %%cr4, %0         \n"
                 "2:                             \n"
                 ".section __ex_table,\"a\"      \n"
                 ".long 1b,2b                    \n"