stand: Export _start on arm like other architectures

By not exporting _start, we get various warnings of the form:

ld: warning: cannot find entry symbol _start

Note that in practice these don't matter because we manually construct
our PE header and use objcopy -O binary, so the entry point is set to
whatever we put explicitly in the PE header, but we should still do the
right thing and silence these warnings.

This was found in CheriBSD, where bsd.prog.mk has similar logic to
bsd.lib.mk and sets -Wl,--(no-)fatal-warnings based on LD_FATAL_WARNINGS
(unlike FreeBSD which only does so in the latter).