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.