x86: Define wrtsc(tsc) as wrmsr(MSR_IA32_TSC, tsc)

Remove some inline assembly code duplication and opportunistically
replace the magic constant, "0x10," with "MSR_IA32_TSC."

Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20220127215548.2016946-3-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>