In -std=gnu* mode, `linux` is a macro that expands to 1. If
LLVM_INTRINSIC_TD_PATH (bare string) contains `linux` (e.g.
x86_64-linux-gnu), LLVM_INTRINSIC_TD_PATH in gen_gccbuiltins.cpp will be
expanded and the whole string will get corrupted.
Fix it by switching from bare strings to C++ raw string literals
(preserving Windows backslashes).