internal/rosa/gnu: gmp explicit CC

The configure script is hard coded to use gcc without fallback on riscv64.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/gnu.go

@@ -1092,7 +1092,11 @@ func (t Toolchain) newGMP() (pkg.Artifact, string) {
 			"gmp-"+version+".tar.bz2",
 		mustDecode(checksum),
 		pkg.TarBzip2,
-	), nil, (*MakeHelper)(nil),
+	), &PackageAttr{
+		Env: []string{
+			"CC=cc",
+		},
+	}, (*MakeHelper)(nil),
 		M4,
 	), version
 }