package/gcc: introduce BR2_GCC_SUPPORTS_LIBCILKRTS

package/gcc: introduce BR2_GCC_SUPPORTS_LIBCILKRTS

libcilkrts was introduced in gcc 4.9, and removed in gcc
8.x. Therefore, it does not make sense to pass --disable-libcilkrts in
gcc 8.x and higher. This commit introduces BR2_GCC_SUPPORTS_LIBCILKRTS
that allows the makefile code to know if the selected gcc version
supports libcilkrts or not.

This new option mainly allows to annotate the fact that libcilkrts
exists in gcc [4.9, 7] and that once we have dropped support for those
gcc versions, the libcilkrts related logic can be removed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>