semihosting: Change internal common-semi interfaces to use CPUState *

This makes all of the internal interfaces architecture-independent and
renames the internal functions to use the 'common_semi' prefix instead
of 'arm' or 'arm_semi'.

To do this, some new architecture-specific internal helper functions
were created:

    static inline target_ulong
    common_semi_arg(CPUState *cs, int argno)

	Returns the argno'th semihosting argument, where argno can be
	either 0 or 1.

    static inline void
    common_semi_set_ret(CPUState *cs, target_ulong ret)

	Sets the semihosting return value.

    static inline bool
    common_semi_sys_exit_extended(CPUState *cs, int nr)

	This detects whether the specified semihosting call, which
	is either TARGET_SYS_EXIT or TARGET_SYS_EXIT_EXTENDED should
	be executed using the TARGET_SYS_EXIT_EXTENDED semantics.

    static inline target_ulong
    common_semi_rambase(CPUState *cs)

	Returns the base of RAM region used for heap and stack. This
	is used to construct plausible values for the SYS_HEAPINFO
	call.

In addition, several existing functions have been changed to flag
areas of code which are architecture specific:

    static target_ulong
    common_semi_flen_buf(CPUState *cs)

	Returns the current stack pointer minus 64, which is
	where a stat structure will be placed on the stack

    #define GET_ARG(n)

	This fetches arguments from the semihosting command's argument
	block. The address of this is available implicitly through the
	local 'args' variable. This is *mostly* architecture
	independent, but does depend on the current ABI's notion of
	the size of a 'long' parameter, which may need run-time checks
	(as it does on AARCH64)

    #define SET_ARG(n, val)

	This mirrors GET_ARG and stores data back into the argument
	block.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20210107170717.2098982-4-keithp@keithp.com>
Message-Id: <20210108224256.2321-15-alex.bennee@linaro.org>