mbuf.gdb: Print mbuf header with p/x
mbuf.gdb: Print mbuf header with p/x

Before this change, these macros used `p` to print the mbuf header.
This produced a lot of garbage for the `om_databuf` member (it is a
`uint8_t*`, so gdb tried to print it as a string).

e.g.,

BEFORE:
    Mbuf header: $527 = {
      om_data = 0x200055b0 <os_msys_1_data+608> "h]",
      om_flags = 0 '\000',
      om_pkthdr_len = 0 '\000',
      om_len = 131,
      om_omp = 0x200068b0 <os_msys_1_mbuf_pool>,
      om_next = {
        sle_next = 0x0
      },
      om_databuf = 0x20005c48 <os_msys_1_data+2296> "Uc\365\271@\362\354\065u\020\354m[\237\320\020\036<bI\235 x$1\252g\221Z\227\071\226e5\344\177\300\374\263\362`t\203\262{\255\343r\370\366\354\352\212S\237Q\355\"b\241\006\253uJ\264\246\334n\366\203\036\\\204\066.Zy\237\t\264\313\066\276\257\347\226\r\017\363=vK0\037\321\217\262\371\257\307\070\320\062\212` 4c\327\273^tA\177\313\362\002\063\033Pu\272\307\226\327\315.\257Ht\205O\006\234\234\253\200T"
    }

AFTER:
    Mbuf header: $534 = {
      om_data = 0x200055b0,
      om_flags = 0x0,
      om_pkthdr_len = 0x0,
      om_len = 0x83,
      om_omp = 0x200068b0,
      om_next = {
        sle_next = 0x0
      },
      om_databuf = 0x20005c48 <os_msys_1_data+2296>
    }

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core