[ARM] Multilib mapping for Armv8-R

[ARM] Multilib mapping for Armv8-R

Due to there being no multilib mapping for Armv8-R, default multilib
builts for -march=armv4t with softfloat floating-point arithmetic is
being used. This patch maps it instead to the existing Armv7 multilibs.
Note that mapping for single-precision Armv8-R has been left out due to
there being no Arm implementation of that architecture variant.

2018-02-22  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/t-multilib: Map Armv8-R to Armv7 multilibs.

    gcc/testsuite/
    * gcc.target/arm/multilib.exp: Add tests for Armv8-R multilib mappings.

From-SVN: r257904