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

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>