selftests/bpf: Fix error linking uprobe_multi on mips

selftests/bpf: Fix error linking uprobe_multi on mips

Linking uprobe_multi.c on mips64el fails due to relocation overflows, when
the GOT entries required exceeds the default maximum. Add a specific CFLAGS
(-mxgot) for uprobe_multi.c on MIPS that allows using a larger GOT and
avoids errors such as:

  /tmp/ccBTNQzv.o: in function `bench':
  uprobe_multi.c:49:(.text+0x1d7720): relocation truncated to fit: R_MIPS_GOT_DISP against `uprobe_multi_func_08188'
  uprobe_multi.c:49:(.text+0x1d7730): relocation truncated to fit: R_MIPS_GOT_DISP against `uprobe_multi_func_08189'
  ...
  collect2: error: ld returned 1 exit status

Fixes: 519dfeaf5119 ("selftests/bpf: Add uprobe_multi test program")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/14eb7b70f8ccef9834874d75eb373cb9292129da.1721692479.git.tony.ambardar@gmail.com