gdbserver: fix Makefile dependency of regformat-generated files on regdat.sh

The intent of the rules modified by this patch is that the *-generated.c
files generated by regdat.sh are re-generated in the event that
regdat.sh is modified.  However, if I build, touch regdat.sh, and build
again, the files are not re-generated during the second build.

This is because regdat.sh is specified as an order-only dependency [1],
after the pipe.  Make therefore only ensures that regdat.sh exists
before generating the target file, it doesn't check the timestamp of
regdat.sh.

This patch changes it to be a regular prerequisite.

The rules use the $< variable, which is substituted by the first
prerequisite only, so the command lines won't change.

[1] https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html

gdb/gdbserver/ChangeLog:

	* Makefile.in (%-generated.c): Make $(regdat_sh) a regular
	prerequisite.