[compiler-rt] Use try_compile_only to check for __ARM_FP

This fixes detection when linking isn't supported (i.e. while building
builtins the first time).

Since 8368e4d5, after setting
CMAKE_TRY_COMPILE_TARGET_TYPE to STATIC_LIBRARY, this isn't strictly
needed, but is good for correctness anyway (and in case that commit
ends up reverted).

Differential Revision: https://reviews.llvm.org/D98737