[ARM] Fix PR85261: ICE with FPSCR setter builtin

[ARM] Fix PR85261: ICE with FPSCR setter builtin

Instruction pattern for setting the FPSCR expects the input value to be
in a register. However, __builtin_arm_set_fpscr expander does not ensure
that this is the case and as a result GCC ICEs when the builtin is
called with a constant literal.

This commit fixes the builtin to force the input value into a register.
It also remove the unneeded volatile in the existing fpscr test and
fixes the function prototype.

2018-04-11  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    PR target/85261
    * config/arm/arm-builtins.c (arm_expand_builtin): Force input operand
    into register.

    gcc/testsuite/
    PR target/85261
    * config/arm/arm-builtins.c (arm_expand_builtin): Force input operand
    into register.

From-SVN: r259310