sysutils/amdmsrtweaker: fix build on recent FreeBSD

bmake has recently started to support $^ in addition to $>, causing
both to expand and leading to a build error like

c++ -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -Wall \
-Werror -pedantic -o amdmsrt Info.o AmdMsrTweaker.o WinRing0.o \
Worker.oInfo.o AmdMsrTweaker.o WinRing0.o Worker.o
c++: error: no such file or directory: 'Worker.oInfo.o'

Fix the error by avoiding both $^ and $>.

Approved by: portmgr (build fix blanket)
MFH: 2026Q1

(cherry picked from commit 87999cd890995b259fa61e70dba80e8a8d153964)