dtrace tests: fix prototypes for gcc build

[rlibby]

Summary

• quiet -Wstrict-prototypes
• provide prototypes for weak aliases

Test Plan

Build with clang and gcc, nm output looks good.

env MAKEOBJDIRPREFIX=/usr/obj/gcc9 CROSS_TOOLCHAIN=amd64-gcc9 make WITH_DTRACE_TESTS=YES buildworld
make WITH_DTRACE_TESTS=YES buildworld

% nm /usr/obj/usr/src/freebsd/amd64.amd64/cddl/usr.sbin/dtrace/tests/common/pid/tst.weak1.o
0000000000000000 W _go
                 U exit
                 U getpid
0000000000000000 T go
0000000000000040 t handle
0000000000000010 T main
                 U signal
% nm /usr/obj/gcc9/usr/src/freebsd/amd64.amd64/cddl/usr.sbin/dtrace/tests/common/pid/tst.weak1.o  
0000000000000000 W _go
                 U exit
                 U getpid
0000000000000000 T go
000000000000000f t handle
000000000000002e T main
                 U signal