Release 5.7.1

This is a bug fix release.

-Tree shaking could cause incorrect errors about undefined symbols when debug information was enabled.

• Expressions that shifted the operand first right then left, or vice versa, could in some situations generate incorrect code when -O2 was used.
• 6502 variants: Spilling a pseudo zero page register and then taking it back to a different location could in some situation result in incorrect register being used.
• 6502 variants: Reading a byte sized volatile and then writing it back to the same location was optimized away at optimization level 1 and above.
• 65816: Reading a byte or 16-bit word sized volatile and then writing it back to the same location was optimized away at optimization level 1 and above.
• 68000: Correct a problem with struct padding that could get wrong if an odd size element was followed by a struct as the last element of the struct.