toolchain.eclass: disable USE=jit for cross-compilers, bug #594572

USE=jit is not very useful for a cross-compiler as it assumes
running generated code on the host. At least avr-* targets
can't build libgccjit.so.

The change disables IUSE=jit for cross-compilers only.
Cross-built compilers could still get libgccjit.so built for a
target.

Reported-by: Maciej Piechotka
Reported-by: Marco Sirabella
Closes: https://bugs.gentoo.org/594572
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

eclass/toolchain.eclass

@@ -2394,6 +2394,10 @@ is_go() {
 
 is_jit() {
 	gcc-lang-supported jit || return 1
+	# cross-compiler does not really support jit as it has
+	# to generate code for a target. On target like avr
+	# libgcclit.so can't link at all: bug #594572
+	is_crosscompile && return 1
 	use_if_iuse jit
 }