package/gdb: update host gdb minimum host gcc version

package/gdb: update host gdb minimum host gcc version

Following gdb 7.12.1 removal [1] the host gcc version needs to be updated
since gdb >= 8.x now requires a C++11 compiler (gcc >= 4.8).

While at it, move BR2_HOST_GCC_AT_LEAST_4_8 under BR2_PACKAGE_HOST_GDB since
it's not an architecture dependency.  Add a comment when the host gcc is too
old to build host gdb.

[1] d36f2c73331b3ab4e238eeb47d7d79d81a895a59

Fixes:
http://autobuild.buildroot.org/results/822/822a747a6717db57705d1ce198a61988aa1173b1

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>