x86: add volatile prefix for cpuid asm clauses

x86: add volatile prefix for cpuid asm clauses

cpuid results are possible to be changed now. For example, changing
CR4.OSXSAVE bit or setting MSR XCR_XFEATURE_ENABLED_MASK may change
XSAVE related cpuid leave return values.

The volatile prefix is required to avoid the second cpuid calls
following some possible changing operations being optimized in
incorrect way by compiler.

The sample bug is in xsave_init while debug=3Dn. The second call to
cpuid_count() may be optimized and lead to a BUG_ON case while compare
xsave_cntxt_size with ebx.

Signed-off-by: Wei Gang <gang.wei@intel.com>