Bump the libm MSRV to 1.67

This gets us:

* `saturating_sub_unsigned`
* `<int>::ilog2`
* Correct lexing of float literals with the `f16` or `f128` suffix

Link: rust-lang/compiler-builtins#1017

Author: tgross35

library/compiler-builtins/libm/Cargo.toml

@@ -13,7 +13,7 @@ keywords = ["libm", "math"]
 repository = "https://github.com/rust-lang/compiler-builtins"
 license = "MIT"
 edition = "2021"
-rust-version = "1.63"
+rust-version = "1.67"
 
 [dev-dependencies]
 # FIXME(msrv): switch to `no-panic.workspace` when possible