MIPS/BFD: Set the ISA bit in microMIPS LA25 stub references

MIPS/BFD: Set the ISA bit in microMIPS LA25 stub references

Fix a linker regression introduced with commit 9d862524f6ae ("MIPS:
Verify the ISA mode and alignment of branch and jump targets") causing a
build failure in microMIPS glibc where the `zdump' tool fails to link:

.../timezone/zdump.o: In function `yeartot':
.../timezone/zdump.c:758:(.text+0x62): Jump to a non-instruction-aligned address
.../timezone/zdump.c:758:(.text+0x76): Jump to a non-instruction-aligned address
.../timezone/zdump.c:768:(.text+0x112): Jump to a non-instruction-aligned address
.../timezone/zdump.c:774:(.text+0x1b8): Jump to a non-instruction-aligned address
.../timezone/zdump.c:774:(.text+0x1cc): Jump to a non-instruction-aligned address
collect2: error: ld returned 1 exit status
make[2]: *** [.../timezone/zdump] Error 1

The cause of the failure is the stricter check introduced with the said
change for jump and branch targets tripping on the address of microMIPS
LA25 stubs.  Despite being microMIPS code these stubs do not have the
ISA bit set throughout the relocation calculation process, because they
have their address set to the memory offset into the stub section they
are placed in.

The `mips_elf_la25_stub' structure does not carry ISA mode information,
but there is no need to extend it, because the ISA mode can be inferred
from the original symbol, which will have STO_MICROMIPS annotation, so
use that instead to set the ISA bit appropriately.  Also only LA25 stubs
associated with microMIPS symbols need to have the ISA bit set, because
other LA25 stubs are made with regular MIPS code, even if associated
with a MIPS16 symbol (in which case they are needed by a call thunk only
rather than the MIPS16 function proper).

	bfd/
	* elfxx-mips.c (mips_elf_calculate_relocation): Set the ISA bit
	in microMIPS LA25 stub references.