x86/boot: Use MSR read/write helpers instead of inline assembly

Update all C code to use the new boot_rdmsr()/boot_wrmsr() helpers
instead of relying on inline assembly.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-7-brijesh.singh@amd.com