arm: armv8: Fix warning about redeclaring global functions as weak

As seen with clang-12:
warning: __asm_invalidate_l3_dcache changed binding to STB_WEAK

As we indeed use ENTRY and then declare the function weak manually.  Use
the WEAK declarative from <linux/linkage.h> instead.

Signed-off-by: Tom Rini <trini@konsulko.com>