Hi,

This patch introduces two IFN's FTRUNC32 and FTRUNC64, the corresponding 
optabs and mappings. It also creates a backend pattern to implement them 
for aarch64 and a match.pd pattern to idiom recognize these.
These IFN's (and optabs) represent a truncation towards zero, as if 
performed by first casting it to a signed integer of 32 or 64 bits and 
then back to the same floating point type/mode.

The match.pd pattern choses to use these, when supported, regardless of 
trapping math, since these new patterns mimic the original behavior of 
truncating through an integer.

I didn't think any of the existing IFN's represented these. I know it's 
a bit late in stage 1, but I thought this might be OK given it's only 
used by a single target and should have very little impact on anything else.

Bootstrapped on aarch64-none-linux.

OK for trunk?

gcc/ChangeLog:

         * config/aarch64/aarch64.md (ftrunc<mode><frintnz_mode>2): New 
pattern.
         * config/aarch64/iterators.md (FRINTZ): New iterator.
         * doc/md.texi: New entry for ftrunc pattern name.
         * internal-fn.def (FTRUNC32): New IFN.
         (FTRUNC64): Likewise.
         * match.pd: Add to the existing TRUNC pattern match.
         * optabs.def (OPTAB_D): New entries for ftrunc.

gcc/testsuite/ChangeLog:

         * gcc.target/aarch64/merge_trunc1.c: Adapted to skip if frintNz 
instruction available.
         * lib/target-supports.exp: Added arm_v8_5a_frintnzx_ok target.
         * gcc.target/aarch64/frintnz.c: New test.