package/gcc: glibc >= 2.27 needs gcc >= 5 on aarch64

package/gcc: glibc >= 2.27 needs gcc >= 5 on aarch64

With the following defconfig:
BR2_aarch64=y
BR2_BINUTILS_VERSION_2_28_X=y
BR2_GCC_VERSION_4_9_X=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y

The build fail with this error:
In file included from ../sysdeps/ieee754/flt-32/math_config.h:23:0,
                 from ../sysdeps/ieee754/flt-32/e_expf.c:29:
../sysdeps/aarch64/fpu/math_private.h: In function 'roundtoint':
../sysdeps/aarch64/fpu/math_private.h:311:3: error: implicit declaration of function 'vrndn_f64' [-Werror=implicit-function-declaration]
   return vget_lane_f64 (vrndn_f64 (vld1_f64 (&x)), 0);
   ^
cc1: all warnings being treated as errors

vrndn_f64 function has been introduced in glibc code since version 2.27 [1].
So we can't build a glibc based toolchain with gcc 4.9 anymore.

Add a new dependency in the toolchain infra to avoid selecting
glibc toolchain using gcc 4.9.

[1] https://sourceware.org/git/?p=glibc.git;a=commit;h=72aa623345ada1276ed89dbc00fdff9639cb8eaf

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>