[ARM] Add support for a set of multilibs to be built for A profile cores.

Hi,

	One of our internal patches is a multilib patch that helps in testing 
bare-metal toolchains for v7-a and above.

It's a bit brute force but this is something that we use internally 
every night to build a set of base libraries in a hierarchical directory 
structure to be used by bare metal toolchains along with those that want 
to do so from a single toolchain. I don't claim to particularly like 
this patch especially the part of writing the MULTILIB_REUSE macros to 
match as many of the options that I could think of that folks would use 
sensibly along with such a toolchain.

- I've missed a few -mfpu options but I deemed them to be uncommon 
enough. If people want to submit a follow on patch for those, I'd be 
happy to take them but this will do for now.

- Additionally if people want to extend this to cover M profile cores 
and all cores, feel free to do so. I don't intend to do so in this cut 
of the makefile fragment.

- I have deliberately avoided documenting that ARM in the supported list 
for --with-multilib-list, because we aren't providing this option like 
other targets in this regard.

- Additionally one cannot use with --with-cpu , --with-fpu, --with-float 
--with-mode --with-arch command line options with this as the make file 
fragment is written assuming such a case.

This does not handle all the general options that --with-multilib-list 
provides with actually providing options on the command line, trying to 
write this with the command lines as suggested in other ports for the 
length that I have in mind, is just going to be unweildy and extremely 
irritating on the command line.

Additionally there is no easy way of supplying multilib_reuse options on 
the command line, with the result that the configure command line would 
be long, prone to error and extremely fragile. This is less fragile than 
that approach and hence I prefer this form for the ARM backend for the 
current work and for people who are interested in this sort of a thing.

Additionally tested with a noddy shell script that goes something like 
and checked for no failures.

for mode in marm mthumb ; do
for fabi in hard softfp  ; do
for fpu in vfpv3-d16 vfpv3 vfpv4-d16 vfpv4 neon neon-vfpv4 fp-armv8 
neon-fp-armv8  ; do
for cpu in cortex-a8 cortex-a5 cortex-a9 cortex-a7 cortex-a15 
cortex-a53; do
   x=`$COMPILER /tmp/hello.c -mcpu="$cpu" -mfpu="$fpu" 
-mfloat-abi="$fabi" "-$mode" -print-multi-directory -lm`
   if [ "$x" != "." ];
   then
       echo "PASS: mcpu=$cpu mfpu=$fpu mfloat-abi=$fabi -$mode"
   else
       echo "FAIL: mcpu=$cpu mfpu=$fpu mfloat-abi=$fabi -$mode"
   fi
done
done
done

for fpu in vfpv3-d16 vfpv3 vfpv4-d16 vfpv4 neon neon-vfpv4 fp-armv8 
neon-fp-armv8 crypto-neon-fp-armv8 ; do
for fabi in hard softfp  ; do
for arch in armv7-a armv8-a ; do
#  $COMPILER /tmp/hello.c -march="$arch" -mfpu="$fpu" 
-mfloat-abi="$fabi" -"$mode" -lm
   x=`$COMPILER /tmp/hello.c -march="$arch" -mfpu="$fpu" 
-mfloat-abi="$fabi" "-$mode" -print-multi-directory -lm`
  if [ "$x" != "." ];
  then
       echo "PASS: march=$arch mfpu=$fpu mfloat-abi=$fabi -$mode"
   else
       echo "FAIL: march=$arch mfpu=$fpu mfloat-abi=$fabi -$mode"
   fi
done
done
done

Tested on arm-none-eabi with --with-multilib-list=aprofile C, C++ and 
Fortran with no regressions on an ARM fast model. Note that this is not 
on by default so will not affect default build times and we have 
auto-testers internally that use this feature.

I think I can apply the changes for config.gcc but I'd still like 
another set of eyes on this please.

Ok ?

regards
Ramana

<DATE>  Matthew Gretton-Dann  <matthew.gretton-dann@arm.com>
	Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

* config/arm/t-aprofile: New file.
* config.gcc: Handle --with-multilib-list option.

[ARM] Add support for a set of multilibs to be built for A profile cores.

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