toolchain: introduce BR2_TOOLCHAIN_HAS_GCC_BUG_63261

toolchain: introduce BR2_TOOLCHAIN_HAS_GCC_BUG_63261

dmalloc and fxload fail to build for the Microblaze architecture with
optimization enabled with gcc < 8.x, with the following failure:

  Error: PC relative branch to label logerror which is not in the instruction space
  Error: operation combines symbols in different segments

The following defconfig allows to reproduce the issue:

BR2_microblazeel=y
BR2_OPTIMIZE_2=y
BR2_KERNEL_HEADERS_5_0=y
BR2_GCC_VERSION_7_X=y
BR2_PACKAGE_FXLOAD=y

The gcc bug was reported at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63261 and is fixed as of
gcc 8.x.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>