Rename the test-multiprecision feature to build-mpfr

Currently the features that control what we test against are
`build-musl` and `test-multiprecision`. I didn't name them very
consistently and there isn't really any reason for that.

Rename `test-multiprecision` to `build-mpfr` to better reflect what it
actually does and to be more consistent with `build-musl`.

library/compiler-builtins/libm/crates/libm-test/src/lib.rs

@@ -5,7 +5,7 @@
 pub mod domain;
 mod f8_impl;
 pub mod gen;
-#[cfg(feature = "test-multiprecision")]
+#[cfg(feature = "build-mpfr")]
 pub mod mpfloat;
 mod num;
 pub mod op;

library/compiler-builtins/libm/crates/libm-test/src/run_cfg.rs

@@ -126,7 +126,7 @@ impl TestEnv {
         let id = ctx.fn_ident;
         let op = id.math_op();
 
-        let will_run_mp = cfg!(feature = "test-multiprecision");
+        let will_run_mp = cfg!(feature = "build-mpfr");
 
         // Tests are pretty slow on non-64-bit targets, x86 MacOS, and targets that run in QEMU. Start
         // with a reduced number on these platforms.