MIPS/BFD: Account for VxWorks .got.plt references

On VxWorks calls can refer directly to a .got.plt entry, in which case
they won't have an entry in the regular GOT.  It was missed with commit
6c42ddb92b90 ("MIPS GOT: Remove TLS GOT info from the symbol table"),
<https://inbox.sourceware.org/binutils/878v6uyaw3.fsf@talisman.default/>,
which revamped GOT entry bookkeeping, and consequently space for regular
GOT entries is needlessly reserved and then not used, with extraneous
null dynamic relocations attached, e.g.:

Relocation section '.rela.dyn' at offset 0x1400 contains 11 entries:
 Offset     Info    Type            Sym.Value  Sym. Name + Addend
00080c0c  00000405 R_MIPS_HI16       00000000   __GOTT_BASE__ + 0
00080c10  00000406 R_MIPS_LO16       00000000   __GOTT_BASE__ + 0
00080c14  00000601 R_MIPS_16         00000000   __GOTT_INDEX__ + 0
0008141c  00000002 R_MIPS_32                    80c5c
00081800  00000002 R_MIPS_32                    80c5c
00081804  00000002 R_MIPS_32                    81800
00081808  00000502 R_MIPS_32         00081808   dglobal + 0
0008180c  00000202 R_MIPS_32         00000000   dexternal + 0
00081428  00000a02 R_MIPS_32         00081c00   x + 0
00000000  00000000 R_MIPS_NONE                  0
00000000  00000000 R_MIPS_NONE                  0

Adjust regular GOT entry counting then to take .got.plt entries into
account, removing a regression from the commit referred:

mips-vxworks  -FAIL: VxWorks shared library test 1
mipsel-vxworks  -FAIL: VxWorks shared library test 1