boost: do not allow selecting fiber module on mips32/mips64

The fiber module uses the cpu_relax() macro, for which the MIPS
implementation uses the "pause" instruction, only available since
mips32r2 and mips64r2. In order to avoid build failures on
mips32/mips64, we disallow the selection of the fiber module for such
architecture variants.

This solution was suggested by Arnout Vandecappelle.

Fixes:

  http://autobuild.buildroot.net/results/0439790b6f472e3d4b2d5431a05aa5b408f62e56/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>