Embedded Swift Example Projects for ARM and RISC-V Microcontrollers

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Excellent! I love how the changes are really minimal, and looking at them, most are pretty self-explanatory (e.g. you needed to add a linker script to produce a correctly laid out ELF file instead of a Mach-O file). I think we should be able to eliminate the need for these stubs:

void posix_memalign(void) {
    while (1) {}
}

void __stack_chk_fail(void) {
    while (1) {}
}

void __stack_chk_guard(void) {
    while (1) {}
}

void free(void *ptr) {
    while (1) {}
}

(1) If you disable stack protectors (-disable-stack-protector swiftc flag), I'd expect __stack_chk_fail and __stack_chk_guard to not be needed anymore. See https://github.com/apple/swift/blob/main/docs/EmbeddedSwift/UserManual.md#external-dependencies. But of course, stack protectors are generally useful.
(2) I think that asking the compiler to place functions into separate sections (-Xfrontend -function-sections, the equivalent of Clang's/GCC's -ffunction-sections) should enable dead-code stripping, and then the posix_memalign and free dependencies will go away because you don't use any allocations in the example. I am actually trying to pursue enabling this behavior by default on ELF, see https://github.com/apple/swift/pull/70788.

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