🐛 Fix _exit implementation

[ion098]

Summary:

Changes the _exit function to flush the serial buffer and indefinitely block.

Motivation:

The _exit function is supposed to never return, but right now it does.

Test Plan:

• Check it compiles
• Test that _exit exits the code
• Test that _exit always flushes the serial buffer

Test Code:

printf("hi");
std::exit(0);
asm volatile("mov r0, #0 \n\tSTR sp, [r0]\n\t"); // if exit does not immediately, there will be a data abort

[WillXuCodes]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[WillXuCodes]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[BennyBot]

@ion098 did you test this already?

[ion098]

@ion098 did you test this already?

No, I haven't been able to test this yet.