memcheck,macos: Fix vbit-test building on macOS x86 architectures. n-i-bz.

memcheck,macos: Fix vbit-test building on macOS x86 architectures. n-i-bz.

Secondary architectures on macOS are generally x86, which requires additional
LDFLAGS to be set to avoid linker errors.

apple clang (clang-800.0.42.1) error:
  ld: illegal text-relocation to '___stderrp' in /usr/lib/libSystem.dylib from '_main'
      in vbit_test_sec-main.o for architecture i386

Fixes: 49ca185 ("Also test memcheck/tests/vbit-test on any secondary arch.")
Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>