GCC 4.8 arm EIGEN_OPTIMIZATION_BARRIER fix (#2315).

Antonio Sánchezrequested to merge
cantonios/eigen:armv7 into master

GCC 4.8 doesn't seem to like the g register constraint, failing to compile with "error: 'asm' operand requires impossible reload".

Tested r instead, and that seems to work, even with latest compilers.

Also fixed some minor macro issues to eliminate warnings on armv7.

Fixes #2315 (closed).

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