x86/AMD: Convert rdmsr_amd_safe() to use asm goto()

x86/AMD: Convert rdmsr_amd_safe() to use asm goto()

Unlike the WRMSR side, we can't use asm goto() unconditionally, because our
toolchain baseline doesn't support asm goto with outputs.

However, the code generation improvements are substantial enough to warrant
the duplicate implementations.

Detect support, and use asm goto() when available.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>