Upgrade to GDB 8.0.

• Commit b5430a3ced fixes parsing of 32-bit MIPS process cores.
• kgdb has been ported to 8.0 including renaming the new OSABI constant, using gdb_bfd_openr, new return type of ldirname, and solib_add changed.
• GDB 8.0 requires C++11, however the port is marked as requiring C++14 as a workaround for bugs in clang 10.3.
• patch-armfbsd is updated for the new OSABI constant names (a.out was removed in 8.0) and new filename convention (arm-fbsd-*)
• patch-gdb-amd64-bsd-nat.c (renamed from patch-gdbamd64-bsd-nat.c) is simplified to just store the r_flags value before collecting registers from the register cache rather than a full copy of the entire register set.
• patch-gdb-corelow.c has been removed. It isn't really correct and if arm cores still cause core dumps the error is in arm-fbsd-tdep.c.
• patch-gdb-i386-fbsd-nat.c renamed to account for rename of patched file.
• patch-gdb-x86bsd-nat.c has been removed. The file was renamed, but the stock file also compiled fine for me on i386.

PR: 221025 (exp-run for 10.3)
Submitted by: luca.pizzamiglio@gmail.com (original version)
Reviewed by: luca.pizzamiglio@gmail.com (maintainer)
Approved by: brooks
Differential Revision: https://reviews.freebsd.org/D11465