Run tests against MPFR on CI where possible

This effectively gives us tests against infinite-precision results on
MacOS and x86+sse Linux.

ci/docker/aarch64-unknown-linux-gnu/Dockerfile

@@ -3,7 +3,7 @@ FROM ubuntu:24.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \
-    gcc-aarch64-linux-gnu libc6-dev-arm64-cross \
+    gcc-aarch64-linux-gnu m4 make libc6-dev-arm64-cross \
     qemu-user-static
 
 ENV TOOLCHAIN_PREFIX=aarch64-linux-gnu-

ci/docker/i686-unknown-linux-gnu/Dockerfile

@@ -2,4 +2,4 @@ FROM ubuntu:24.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
-    gcc-multilib libc6-dev ca-certificates
+    gcc-multilib m4 make libc6-dev ca-certificates

ci/docker/x86_64-unknown-linux-gnu/Dockerfile

@@ -2,4 +2,4 @@ FROM ubuntu:24.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
-    gcc libc6-dev ca-certificates
+    gcc m4 make libc6-dev ca-certificates

ci/run.sh

@@ -24,6 +24,22 @@ case "$target" in
     *thumb*) extra_flags="$extra_flags --exclude musl-math-sys" ;;
 esac
 
+# Configure which targets test against MPFR
+case "$target" in
+    # MSVC cannot link MPFR
+    *windows-msvc*) ;;
+    # FIXME: MinGW should be able to build MPFR, but setup in CI is nontrivial.
+    *windows-gnu*) ;;
+    # Targets that aren't cross compiled work fine
+    # FIXME(ci): we should be able to enable aarch64 Linux here once GHA
+    # support rolls out.
+    x86_64*) extra_flags="$extra_flags --features libm-test/multiprecision-tests" ;;
+    # i686 works fine, i586 does not
+    i686*) extra_flags="$extra_flags --features libm-test/multiprecision-tests" ;;
+    # Apple aarch64 is native
+    aarch64*apple*) extra_flags="$extra_flags --features libm-test/multiprecision-tests" ;;
+esac
+
 # FIXME: `STATUS_DLL_NOT_FOUND` testing macros on CI.
 # <https://github.com/rust-lang/rust/issues/128944>
 case "$target" in