aarch64: Make the test for available cached PCS data more robust.

aarch64: Make the test for available cached PCS data more robust.

When we are emitting MI thunks, it can be the case that the
target function of the thunk has already been compiled, and
considered complete, with at least part of the relevant data
freed.

The deal with this, we amend the test for the availability
of the cached PCS value to check for the presence of both the
cfun and the machine content.

Since these functions are in a hot code path, use
gcc_checking_assert().

gcc/ChangeLog:

	* config/aarch64/aarch64.cc
	(aarch64_function_abi): Use checking assert.
	(aarch64_fndecl_abi): Test for the availability of the
	machine content before trying to look up the cached PCS.
	Use a checking assert.

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