toolchain.eclass: only enable JIT with USE=jit

My fix in 6335bd10df exposed another
issue with USE="-jit libgdiagnostics" that was being obscured by the
issue I fixed in that commit: with USE=-jit, as long as we were still
doing the separate host-shared build (with USE=libgdiagnostics), we were
still passing --enable-languages=jit.

Bug: https://bugs.gentoo.org/953823
Closes: https://bugs.gentoo.org/954077
Signed-off-by: Sam James <sam@gentoo.org>

eclass/toolchain.eclass

@@ -1915,7 +1915,6 @@ toolchain_src_configure() {
 			--disable-systemtap
 
 			--enable-host-shared
-			--enable-languages=jit
 
 			# Might be used for the just-built GCC. Easier to just
 			# respect USE=graphite here in case the user passes some
@@ -1924,6 +1923,12 @@ toolchain_src_configure() {
 			--with-system-zlib
 		)
 
+		if is_jit ; then
+			confgcc_jit+=( --enable-languages=jit )
+		else
+			confgcc_jit+=( --enable-languages=c,c++ )
+		fi
+
 		if tc_has_feature zstd ; then
 			confgcc_jit+=( $(use_with zstd) )
 		fi