BaseTools/tools_def ARM: use softfloat target for CLANG3x

BaseTools/tools_def ARM: use softfloat target for CLANG3x

The 'arm-linux-gnueabihf' target triplet we use for CLANG35 and
CLANG38 specifies a hardfloat target, and so the binaries that are
emitted are annotated as using VFP registers for passing floating
point arguments, even though no VFP is used anywhere in the code.

This works fine as long as we don't try to link against code
that uses software floating point, but combining object files
with different floating point calling conventions is not permitted.

So switch to the softfloat arm-linux-gnueabi triplet instead.
This affects both the name Clang uses when invoking the linker,
and the arguments it passes to it, and we are mostly interested
in the latter (since any version of GNU ld.bfd will do the right
thing as long as it targets EABI ARM)

For native builds, this change has no effect, since the unprefixed
system linker will take priority, and so Clang will pass the right
arguments to whichever linker happens to be the system linker.

For cross builds, the fact that Clang composes the name of the
linker by prefixing '-ld' with the target triplet implies that
users will have to switch to a version of binutils that targets
arm-linux-gnueabi rather than arm-linux-gnueabihf. Note that the
GCCx toolchain targets can use either when building for ARM so this
does not create a need to install two versions of the ARM cross
toolchain. Also, note that all ARM toolchains in the GCC family
are already documented as requiring a toolchain that targets
arm-linux-gnueabi and not arm-linux-gnueabihf.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>