MIPS16/GAS: Rename the LONG_BRANCH relaxation flag

MIPS16/GAS: Rename the LONG_BRANCH relaxation flag

Following commit 177b4a6ad004 ("infinite loop in mips16 assembler
relaxation"), <https://sourceware.org/ml/binutils/2002-03/msg00345.html>
the LONG_BRANCH flag used in MIPS16 relaxation has lost its use for
branches.  Complement commit 88a7ef168927 ("MIPS16/GAS: Restore
unsupported relocation diagnostics") then, which has removed the remains
of code deactivated by the former commit, and rename the flag to
ALWAYS_EXTENDED, more accurately reflecting its current use to select
the extended form of PC-relative ADDIU, DADDIU, LD and LW instructions.

	gas/
	* config/tc-mips.c (RELAX_MIPS16_LONG_BRANCH): Rename to...
	(RELAX_MIPS16_ALWAYS_EXTENDED): ... this.
	(RELAX_MIPS16_MARK_LONG_BRANCH): Rename to...
	(RELAX_MIPS16_MARK_ALWAYS_EXTENDED): ... this.
	(RELAX_MIPS16_CLEAR_LONG_BRANCH): Rename to...
	(RELAX_MIPS16_CLEAR_ALWAYS_EXTENDED): ... this.
	(mips16_extended_frag): Adjust accordingly.