Skip aeabi_cdcmp and aeabi_cfcmp on big-endian targets

[paoloteti]

First of all aeabi_cdcmp and aeabi_cfcmp are never called LLVM.
Second are little-endian only, so build fail on big-endian targets as show below:

cargo:warning=./compiler-rt/lib/builtins/arm/aeabi_cdcmp.S:13:2: error: #error big endian support not implemented
cargo:warning= #error big endian support not implemented
cargo:warning=  ^~~~~

Temporally workaround: exclude these files for big-endian targets.

Tested on a Texas Instruments TMS570LS3137 (Cortex-R4F big-endian) based board.

[alexcrichton]

Thanks! Looks like there may be a build failure though?

[alexcrichton]

Thanks!