Eight more jit backports to gcc 5 branch

[David Malcolm <dmalcolm at redhat dot com>]

I've gone over the changes to the gcc/jit and gcc/testsuite/jit.dg
directories in trunk since the last backports to gcc 5 [1] and
backported the following 8 changes from trunk to the gcc-5-branch:

gcc-5-branch's r225251:
  * trunk's r225203 (9bd7a189c372465fb757ecd67336379779f4ea60,
"jit: fix some .rst issues in docs")
    https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02248.html

gcc-5-branch's r225252:
  * trunk's r225204 (cc36aead6a4e5017477c9c0836672317128ed6b3,
"fixes to gcc_jit_context_dump_reproducer_to_file")
    https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02249.html

gcc-5-branch's r225253:
  * trunk's r225205 (adb2df5592cdf8e70aa44c0f3c447da1d0134f4c,
"PR jit/66628: add gcc_jit_context_add_command_line_option")
    https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02250.html

gcc-5-branch's r225254:
  * trunk's r225206 (04feb56e6acd497d0add042232afd7940ef61adb,
"PR jit/66546: Add gcc_jit_context_set_bool_allow_unreachable_blocks")
    https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02251.html

gcc-5-branch's r225255:
  * trunk's r225207 (a24ef8d231d1015c5a4ee68050ed78f5582342fe,
"jit: add switch statements")
    https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02256.html

gcc-5-branch's r225256:
  * trunk's r225245 (1fa42b0566972abc668f3aa88e5534092af99c2c,
"jit: clarify (lack of) lifetime requirements on input const char *")
    https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00043.html

gcc-5-branch's r225257:
  * trunk's r225246 (eb2d3e4a67c699257e42280e6bf800b8a92ffad7,
"jit: document union types")
    https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00045.html

gcc-5-branch's r225258:
  * trunk's r225248 (f410e1f5b44007c6cf6609df0afc16e7dcfeabd2,
"PR jit/66700: set TREE_ADDRESSABLE when building an ADDR_EXPR")
    https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00046.html

"make check-jit" on gcc-5-branch went from 8014 to 8289 passes.

In particular, note that this adds symbol versioning to libgccjit.so on
the branch to track (so far) the changes made on trunk, adding some API
entrypoints needed by users (for PR jit/66546 and PR jit/66700).

Dave

[1] ec2e0095a3a5988b03a2706b5ffe0e807b238ba8 (on 2015-06-25)