mips-tdep.c: Remove MAX_REGISTER_SIZE usage

mips-tdep.c: Remove MAX_REGISTER_SIZE usage

mips_eabi_push_dummy_call is storing the address of a struct in a
buffer that must have the same of the confisued/set ABI register size.
Add a define for the maximum ABI size and use it to size the local
buffer.  Also rename the 'regsize' local to 'abi_regsize' for clarity.

Tested that --enable-targets=all still builds.

gdb/ChangeLog:
2017-06-12  Pedro Alves  <palves@redhat.com>
	    Alan Hayward  <alan.hayward@arm.com>

	* mips-tdep.c (MAX_MIPS_ABI_REGSIZE): New.
	(mips_eabi_push_dummy_call): Rename local 'regsize' to
	'abi_regsize'.  Rename local array 'valbuf' to 'ref_valbuf', and
	use MAX_MIPS_ABI_REGSIZE instead of MAX_REGISTER_SIZE to size it.
	Assert that abi_regsize bytes fit in 'ref_valbuf'.