Ensure that the jump buffer is appropriately aligned.

[gareth-rees]

The stack capturing mechanism in STACK_CONTEXT_BEGIN puts the StackContextStruct on the stack, where it will later be scanned using TraceScanArea. The jumpBuffer must be allocated on the stack at an address with suitable alignment so that TraceScanArea will correctly fix any addresses therein. On platform XCA6LL, jmp_buf is declared as an array of int, which has 4-byte alignment, but we need it to have 8-byte alignment in order to correctly interpret the contents.

This addresses fifteen of the seventeen MMQA test failures in issue #84.

[thejayps]

@rptb1 : blocked because we are waiting for Apple Silicon CI to be available. This will come from #145.