librdmacm/libibverbs: Add bnxtre RDMA provider to OFED build infrastructure

Extend the FreeBSD OFED build framework to include the Broadcom bnxtre RDMA
provider library.

This change:

-Registers libbnxtre in src.libnames.mk so it is built and installed as part

of the OFED libraries.

-Adds bnxtre to the OFED pcap dependency set when MK_OFED is enabled.
-Declares proper dependency mappings for bnxtre (ibverbs, pthread).
-Introduces LIBBNXTREDIR for consistent object directory handling.
-Updates libibverbs and librdmacm build rules to link against libbnxtre,

enabling Broadcom RoCE device support at runtime.

-libbnxtre library uses below constructor to register with the libibverbs,

static attribute((constructor)) void bnxt_re_register_driver(void)

These updates ensure that applications using libibverbs and librdmacm can
discover and use Broadcom bnxt RDMA devices on FreeBSD.

Reviewed by: markj, ssaxena
Differential Revision: https://reviews.freebsd.org/D54368
MFC after: 3 days