toolchain/toolchain-external/toolchain-external-synopsys-arc: fix condition

The option BR2_TOOLCHAIN_EXTERNAL_SYNOPSYS_ARC just depends on
BR2_arc, but the choice of toolchains that appears once
BR2_TOOLCHAIN_EXTERNAL_SYNOPSYS_ARC=y only has choices for:

	depends on BR2_arc750d || BR2_arc770d

and

	depends on BR2_archs38_64mpy || BR2_archs38_full || \
		BR2_archs4x_rel31 || BR2_archs4x

which means that if you have a BR2_archs38=y configuration, you end up
with no choice of toolchain, and the build fails with "No C library
selected". Fix this by making sure BR2_TOOLCHAIN_EXTERNAL_SYNOPSYS_ARC
can only be selected when an actual toolchain is available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

toolchain/toolchain-external/toolchain-external-synopsys-arc/Config.in

@@ -1,6 +1,8 @@
 config BR2_TOOLCHAIN_EXTERNAL_SYNOPSYS_ARC
 	bool "Synopsys ARC 2024.06 toolchain"
-	depends on BR2_arc
+	depends on BR2_arc750d || BR2_arc770d || \
+		BR2_archs38_64mpy || BR2_archs38_full || \
+		BR2_archs4x_rel31 || BR2_archs4x
 	depends on BR2_HOSTARCH = "x86_64"
 	depends on !BR2_ARCH_NEEDS_GCC_AT_LEAST_15
 	select BR2_INSTALL_LIBSTDCPP