tools/rv: Keep user LDFLAGS in build

tools/rv: Keep user LDFLAGS in build

rv, unlike rtla and perf, drops LDFLAGS supplied by the user and honors
only EXTRA_LDFLAGS. This is inconsistent with both perf and rtla and
can lead to all kinds of unexpected behavior.

For example, on Fedora and RHEL, it causes rv to be build without
PIE, unlike the aforementioned perf and rtla:

$ file /usr/bin/{rv,rtla,perf}
/usr/bin/rv:   ELF 64-bit LSB executable, ...
/usr/bin/rtla: ELF 64-bit LSB pie executable, ...
/usr/bin/perf: ELF 64-bit LSB pie executable, ...

Keep both LDFLAGS and EXTRA_LDFLAGS for the build.

Cc: John Kacur <jkacur@redhat.com>
Cc: Luis Goncalves <lgoncalv@redhat.com>
Cc: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/20250304142228.767658-1-tglozar@redhat.com
Fixes: 012e4e77df73 ("tools/verification: Use tools/build makefiles on rv")
Signed-off-by: Tomas Glozar <tglozar@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>