tcc_enter/exit_state() are meant exclusively to protect
the tcc_compile() and its sub-functions in tccpp.c,
tccgen.c, tccasm.c and xxx-gen.c.

Other files that are part of libtcc simply must not use global
variables.

- riscv64/last_hi: move to TCCState
  from 72250bec

- tccrun.c: Using a fixed address would not work anyway
  ("tcc -run tcc.c -run ..." for example)
  from baacb0f5

- tests/Makefile: support for a platform doesn't make sense if
  it doesn't pass our basic tests.
  from 591feda1
Also:
- tccgen: cleanup "duplicate member" (only 2 passes,
  avoids additional TokenSym field)
  from 170be79a