BACKPORT: arm64: Change .weak to SYM_FUNC_START_WEAK_PI for arch/arm64/lib/mem*.S

BACKPORT: arm64: Change .weak to SYM_FUNC_START_WEAK_PI for arch/arm64/lib/mem*.S

commit ec9d78070de986ecf581ea204fd322af4d2477ec upstream.

Commit 39d114ddc682 ("arm64: add KASAN support") added .weak directives to
arch/arm64/lib/mem*.S instead of changing the existing SYM_FUNC_START_PI
macros. This can lead to the assembly snippet `.weak memcpy ... .globl
memcpy` which will produce a STB_WEAK memcpy with GNU as but STB_GLOBAL
memcpy with LLVM's integrated assembler before LLVM 12. LLVM 12 (since
https://reviews.llvm.org/D90108) will error on such an overridden symbol
binding.

Use the appropriate SYM_FUNC_START_WEAK_PI instead.

Fixes: 39d114ddc682 ("arm64: add KASAN support")
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201029181951.1866093-1-maskray@google.com
Signed-off-by: Will Deacon <will@kernel.org>
[nd: backport to adjust for missing:
  commit 3ac0f4526dfb ("arm64: lib: Use modern annotations for assembly functions")
  commit 35e61c77ef38 ("arm64: asm: Add new-style position independent function annotations")]
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: Ibebbfa15f3337b5a2ac88ba683c5e429758d7a98