devel/llvm12: address build issues in some configs

In some cases (notably when LLDB is not selected), COMPILER_RT fails to
build owing to not finding unwind.h. Add USES=localbase to COMPILER_RT
to address this.[0]

When building on amd64 without /usr/lib32 compat libraries installed,
COMPILER_RT doesn't build i386 sanitizer libs. Detect this case and
don't add to plist.[1]

Submitted by: Tatsuki Makino [0]
PR: 254048 [0], 255527 [1]
MFH: 2020Q2