target-mips: Add nan2008 flavor of <CEIL|CVT|FLOOR|ROUND|TRUNC>.<L|W>.<S|D>

New set of helpers for handling nan2008-syle versions of instructions
<CEIL|CVT|FLOOR|ROUND|TRUNC>.<L|W>.<S|D>, for Mips R6.

All involved instructions have float operand and integer result. Their
core functionality is implemented via invocations of appropriate SoftFloat
functions. The problematic cases are when the operand is a NaN, and also
when the operand (float) is out of the range of the result.

Here one can distinguish three cases:

CASE MIPS-A: (FCR31.NAN2008 == 1)

   1. Operand is a NaN, result should be 0;
   2. Operand is larger than INT_MAX, result should be INT_MAX;
   3. Operand is smaller than INT_MIN, result should be INT_MIN.

CASE MIPS-B: (FCR31.NAN2008 == 0)

   1. Operand is a NaN, result should be INT_MAX;
   2. Operand is larger than INT_MAX, result should be INT_MAX;
   3. Operand is smaller than INT_MIN, result should be INT_MAX.

CASE SoftFloat:

   1. Operand is a NaN, result is INT_MAX;
   2. Operand is larger than INT_MAX, result is INT_MAX;
   3. Operand is smaller than INT_MIN, result is INT_MIN.

Current implementation of <CEIL|CVT|FLOOR|ROUND|TRUNC>.<L|W>.<S|D>
implements case MIPS-B. This patch relates to case MIPS-A. For case
MIPS-A, only return value for NaN-operands should be corrected after
appropriate SoftFloat library function is called.

Related MSA instructions FTRUNC_S and FTINT_S already handle well
all cases, in the fashion similar to the code from this patch.

Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
[leon.alrae@imgtec.com:
 * removed a statement from the description which caused slight confusion]
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>

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