x86_no_sse
Emit `x86_no_sse` in the compiler-builtins (and builtins-test) build script, and use it to simplify `all(target_arch = "x86", not(target_fefature = "sse))` configuration.
__powitf2
testcrate
builtins-test
The repo will soon have `libm` as a top-level crate, so make it clear that this is only the test crate for `compiler-builtins`.