Darwin: Fixes for darwin_libc_has_function.

Darwin: Fixes for darwin_libc_has_function.

Firstly, the checks for availability need not be run for any
currently supported Darwin version (or for any version of
Darwin on x86).  In fact, the only test that is needed that
differs from the default is for the availbaility of sincos.
Test that and then fall back to the default implementation.

Secondly, the funtion appears to be called from the Jit library
before the value of darwin_macosx_version_min has been set up -
at present we work around this by guarding the checks on having
a non-null pointer for darwin_macosx_version_min.

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

	* config/darwin.c (darwin_libc_has_function): Do not run
	the checks for x86 or modern Darwin.  Make sure that there
	is a value set for darwin_macosx_version_min before testing.