[Driver] Incorporate -mfloat-abi in the computed triple on ARM

[john.brawn]

Summary

LLVM assumes that when it creates a call to a C library function it can use the C calling convention. On ARM the effective calling convention is determined from the target triple, however using -mfloat-abi=hard on ARM means that calls to (and definitions of) C library functions use the arm_aapcs_vfpcc calling convention which can result in a mismatch.

Fix this by incorporating -mfloat-abi into the target triple, similar to how -mbig-endian and -march/-mcpu are. This only works for EABI targets and not Android or iOS, but there the float abi is fixed so instead give an error.

Fixes PR45524.

[efriedma]

clang/lib/Driver/ToolChain.cpp

807

Do we want to error in the "default" case? Not that anyone is likely to use -mfloat-abi on those targets, but I'd prefer not to silently miscompile.

[john.brawn]

Adjusted to give an error when the specified float abi is incompatible with the default. Was a bit trickier than I expected as it required extracting out the logic to determine the default into a separate function (we could instead emit the error in arm::getARMFloatABI but then it would be emitted several times as the function is called from several places).

[efriedma]

LGTM