toolchain/toolchain-common.in: add BR2_TOOLCHAIN_HAS_LIBQUADMATH hidden symbol

This hidden symbol allow to know when libquadmath can be built and
installed.

Also, declaring this symbol in toolchain-common.in allows to use it in
both external and buildroot toolchain backend.

This will be needed for adding/improving the fortran support.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/toolchain-common.in

@@ -419,3 +419,13 @@ config BR2_TOOLCHAIN_HAS_ATOMIC
 	default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_armeb
 	default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_xtensa
 	default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_ARCH_IS_64
+
+# - libquadmath is not needed/available on all architectures (but gcc
+#   correctly handles this already).
+# - At least, libquadmath is available on:
+#   - i*86
+#   - x86_64
+# - When available, libquadmath requires wchar support.
+config BR2_TOOLCHAIN_HAS_LIBQUADMATH
+	bool
+	default y if BR2_i386 || BR2_x86_64