Included more math functions to Windows's entrypoints
and made a cmake option (-DLLVM_LIBC_MPFR_INSTALL_PATH)
where the user can specify the install path where the MPFR
library was built so it can be linked. The try_compile was
moved to LLVMLibCCheckMPFR.cmake, so the variable that is
set after this process can retain its value in other files
of the same parent file. A direct reason for this is for
LIBC_TESTS_CAN_USE_MPFR to be true when the user specifies
MPFR's path and retain its value even after leaving the file.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D106894
Hedin Garca