jit, Darwin: Implement library exports list.

jit, Darwin: Implement library exports list.

Currently, we have no exports list for libgccjit, which means that
all symbols are exported, including those from libstdc++ which is
linked statically into the lib.  This causes failures when the
shared libstdc++ is used but some c++ symbols are satisfied from
libgccjit.

This implements an export file for Darwin (which is currently
manually created by cross-checking libgccjit.map).  Ideally we'd
script this, at some point.  Update libtool current and age to
reflect the current ABI version (we are not bumping the SO name
at this stage).

This fixes a number of new failures in jit testing.

gcc/jit/ChangeLog:

	* Make-lang.in: Implement exports list, and use a shared
	libgcc.
	* libgccjit.exports: New file.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>