do boundary check for MCContext::Instances[]. this fixes crash-01-x64-empty-tombstone-value-shouldnt-be-inserted-into-map.c

sanity check on number of bits of FP in ARM, AArch64 & X86 parsers. this fixes crash-27-x64-too-many-bits-for-uint64_t.c

handle equal directive. this fixes c-crashes/crash-12-x64-cannot-set-a-variable-that-has-already-been-used.c

handle error in applyFixup(). this fixes c-crashes/crash-23-x64-value-does-not-fit-in-the-fixup-field.c

x86: report unsupported instructions in encodeInstruction(). this fixes c-crashes/crash-24-x64-not-is16bitmemoperand.c

This breaks suite/regress/x64_nasm_default_rel.py.

arm: propagate error from ARMAsmParser::validateTargetOperandClass(). this fixes crash-16-expression-value-must-be-representable-in-32-bits.c

propagate error from APFloat::convertFromDecimalString(). this fixes crash-06-exponent-has-no-digits-in-apfloat-line-126.c

mips: propagate error from MipsAsmParser::processInstruction() back to ks_asm(). this fixes issue #110

Fixed relative address encoding for branches at ARM and Thumb mode.
Also added correct checks for out of range branches. Finally removed
a check for Thumb mode that would never succeed.