[compiler-rt] Allow building builtins.a without a libc (#86737)

compiler-rt may depend on libc (memset etc). Likewise a libc built by
clang may depend on compiler-rt builtins.

This circular dependency doesn't matter much once they're both compiled.
The easy compilation order to build both from source is:

1. install libc headers somewhere
2. build compiler-rt builtins against those headers
3. build libc against compiler-rt builtins

This patch relaxes the cmake sanity check to pass without requiring a
libc library. That allows the above sequence to work. Otherwise one
needs to build a static libc, then use that to pass the compiler-rt
cmake check, then build a normal libc.