In usr.bin/rpcgen/rpc_main.c, use execvp(3) instead of execv(3), so rpcgen will search the current PATH for the preprocessor.  This makes it possible to run a preprocessor built during the cross-tools stage of buildworld.
In usr.bin/rpcgen/rpc_main.c, use execvp(3) instead of execv(3), so
rpcgen will search the current PATH for the preprocessor.  This makes it
possible to run a preprocessor built during the cross-tools stage of
buildworld.

MFC after:	1 week

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