[NEWLIB][Cross-build]: Fix merge-newlib-nano.

There is no $(NEWLIB_TUPLE)-gcc in install directory
during cross-compilation process. For example,
to compile for Windows, we has to have riscv64-unknown-elf-gcc
in our PATH and we has to use it with print-multi-lib
to find multilib configuration, because install
folder contains only .exe files (can not be executed)

This bugfix is to check: whether compiler we already
have for host - in the install directory or in the PATH.