gcc: xtensa: add -mabi option for call0/windowed ABI

2020-06-15  Max Filippov  <jcmvbkbc@gmail.com>
gcc/
	* config/xtensa/elf.h (ASM_SPEC, LINK_SPEC): Pass ABI switch to
	assembler/linker.
	* config/xtensa/linux.h (ASM_SPEC, LINK_SPEC): Ditto.
	* config/xtensa/uclinux.h (ASM_SPEC, LINK_SPEC): Ditto.
	* config/xtensa/xtensa.c (xtensa_option_override): Initialize
	xtensa_windowed_abi if needed.
	* config/xtensa/xtensa.h (TARGET_WINDOWED_ABI_DEFAULT): New
	macro.
	(TARGET_WINDOWED_ABI): Redefine to xtensa_windowed_abi.
	* config/xtensa/xtensa.opt (xtensa_windowed_abi): New target
	option variable.
	(mabi=call0, mabi=windowed): New options.
	* doc/invoke.texi: Document new -mabi= Xtensa-specific options.

gcc/testsuite/
	* gcc.target/xtensa/mabi-call0.c: New test.
	* gcc.target/xtensa/mabi-windowed.c: New test.

libgcc/
	* configure: Regenerate.
	* configure.ac: Use AC_COMPILE_IFELSE instead of manual
	preprocessor invocation to check for __XTENSA_CALL0_ABI__.