Escape LLVM_TARGETS_TO_BUILD while checking against LLVM_ALL_TARGETS and...

Escape LLVM_TARGETS_TO_BUILD while checking against LLVM_ALL_TARGETS and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD (#70885)

Targeting X86 while building LLVM with the Android NDK, currently fails
with "The target `X86' is not a core tier target. It may be experimental,
if so it must be passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD."

This is happening because the Android NDK defines a variable named X86,
which will lead to a "fun" CMake feature. For further details reference
[this post](https://discourse.llvm.org/t/the-target-x86-is-not-a-core-tier-target/73784).

To fix this, the LLVM_TARGETS_TO_BUILD need to be escaped while checking
agains LLVM_ALL_TARGETS and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.