Makefile: Replace shell functions with built-in equivalents
Makefile: Replace shell functions with built-in equivalents

Wherever possible, rely on facilities native to GNU make rather than
invoke external utilities superfluously.  Change the shell to /bin/sh
instead of bash since the latter's heftier feature set is unnecessary.

The -r option to cp(1) is marked obsolescent by POSIX; use -R instead.

manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain