gcc: only use save/restore routines if they don't increase stack usage
gcc: only use save/restore routines if they don't increase stack usage

When a function clobbers only s0 and s1, but not ra, we don't want to use
the save/restore routines because the stack would grow more than needed.

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