i386: Cleanup i386/i386elf.h and align it's return convention with the SVR4 ABI

i386: Cleanup i386/i386elf.h and align it's return convention with the SVR4 ABI

While i386elf.h was originally derived from sysv4.h it has not been kept
up to date with the development of the compiler. Two changes are made:

* The return convention now follows the i386 and x86_64 SVR4 ABIs again.

* The more efficient default version of ASM_OUTPUT_ASCII in elfos.h is used.

2020-11-04  Pat Bernardi  <bernardi@adacore.com>

gcc/ChangeLog

	* config/i386/i386elf.h (SUBTARGET_RETURN_IN_MEMORY): Remove.
	(ASM_OUTPUT_ASCII): Likewise.
	(DEFAULT_PCC_STRUCT_RETURN): Define.
	* config/i386/i386.c (ix86_return_in_memory): Remove
	SUBTARGET_RETURN_IN_MEMORY.