[PPCAsmPrinter] support 'L' output template for memory operands

Summary:
L is meant to support the second word used by 32b calling conventions for 64b arguments.

This is required for build 32b PowerPC Linux kernels after upstream
commit 334710b1496a ("powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'")

Thanks for the report from @nathanchance, and reference to GCC's
implementation from @segher.

Fixes: pr/46186
Fixes: https://github.com/ClangBuiltLinux/linux/issues/1044

Reviewers: echristo, hfinkel, MaskRay

Reviewed By: MaskRay

Subscribers: MaskRay, wuzish, nemanjai, hiraditya, kbarton, steven.zhang, llvm-commits, segher, nathanchance, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81767