gas: rename md_end to md_finish

Currently md_end is typically used for some final actions rather than
freeing memory like other *_end functions.  Rename it to md_finish,
and rename target implementation.  The renaming of target functions
makes it possible to find them all with "grep md_finish",
eg. md_mips_end is renamed to mips_md_finish, not md_mips_finish.
This patch leaves a number of md_end functions unchanged, those that
either do nothing or deallocate memory, and calls them late.

The idea here is that target maintainers implement md_end functions to
tidy memory, if anyone cares.  Freeing persistent memory in gas is
not at all important, except that it can hide more important memory
leaks, those that happen once per some frequent gas operation, amongst
these unimportant memory leaks.

	* as.c (main): Rename md_end to md_finish.
	* config/tc-alpha.c, * config/tc-alpha.h,
	* config/tc-arc.c, * config/tc-arc.h,
	* config/tc-arm.c, * config/tc-arm.h,
	* config/tc-csky.c, * config/tc-csky.h,
	* config/tc-ia64.c, * config/tc-ia64.h,
	* config/tc-mcore.c, * config/tc-mcore.h,
	* config/tc-mips.c, * config/tc-mips.h,
	* config/tc-mmix.c, * config/tc-mmix.h,
	* config/tc-msp430.c, * config/tc-msp430.h,
	* config/tc-nds32.c, * config/tc-nds32.h,
	* config/tc-ppc.c, * config/tc-ppc.h,
	* config/tc-pru.c, * config/tc-pru.h,
	* config/tc-riscv.c, * config/tc-riscv.h,
	* config/tc-s390.c, * config/tc-s390.h,
	* config/tc-sparc.c, * config/tc-sparc.h,
	* config/tc-tic4x.c, * config/tc-tic4x.h,
	* config/tc-tic6x.c, * config/tc-tic6x.h,
	* config/tc-v850.c, * config/tc-v850.h,
	* config/tc-xtensa.c, * config/tc-xtensa.h,
	* config/tc-z80.c, * config/tc-z80.h: Similarly.
	* output-file.c (output_file_close): Call md_end.