x86/MCE/AMD: Split amd_mce_is_memory_error()

Define helper functions for legacy and SMCA systems in order to reuse
individual checks in later changes.

Describe what each function is checking for, and correct the XEC bitmask
for SMCA.

No functional change intended.

  [ bp: Use "else in amd_mce_is_memory_error() to make the conditional
    balanced, for readability. ]

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Shuai Xue <xueshuai@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230613141142.36801-2-yazen.ghannam@amd.com