PCI: Fix PREL32 relocations for LTO

With Clang's Link Time Optimization (LTO), the compiler can rename
static functions to avoid global naming collisions. As PCI fixup
functions are typically static, renaming can break references
to them in inline assembly. This change adds a global stub to
DECLARE_PCI_FIXUP_SECTION to fix the issue when PREL32 relocations
are used.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201211184633.3213045-10-samitolvanen@google.com