x86/mm: Unify __phys_addr_symbol()

x86/mm: Unify __phys_addr_symbol()

There are two implementations on 64-bit, depending on CONFIG_DEBUG_VIRTUAL,
but they differ only regarding the presence of VIRTUAL_BUG_ON, which is
already ifdef'd on CONFIG_DEBUG_VIRTUAL.

To avoid adding a function call on non-LTO non-DEBUG_VIRTUAL builds, move the
function into the header. (Note the function is already only used on 64-bit).

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/all/20250813-phys-addr-cleanup-v1-1-19e334b1c466@google.com/