cortexm: optimize code size for the HardFault_Handler

This function is called when a hard fault occurs. Hard faults happen
when something really bad happens - like writing to unwritable memory or
an unaligned memory access on Cortex-M0. It is not generally possible to
recover from these.

This commit optimizes the code size overhead of hard fault handling:

  * It removes the stack overflow checking code.
    This may seem like a bad thing, but the only thing this could check
    were stack overflows outside goroutines. In practice, this could
    only really happen on a stack overflow in the scheduler (unlikely),
    or in interrupt code (possible, but interrupts are small so still
    unlikely). Most stack overflows happen in regular goroutines, and
    weren't caught in the HardFault.
  * It makes the panic message similar to a regular panic. This has two
    advantages:
      * It reduces code size, because the string can be reused between
        the HardFault handler and the runtime panic function.
      * Using the same pattern automatically makes `-monitor` print the
        source address for the hard fault. Not a big benefit as we could
        trivially add any other pattern but a nice benefit nonetheless.

Result:

    $ tinygo flash -target=microbit -size=short -programmer=openocd -monitor examples/serial
       code    data     bss |   flash     ram
       3036       8    2256 |    3044    2264
    [...snip]
    Connected to /dev/ttyACM0. Press Ctrl-C to exit.
    panic: runtime error at 0x00000344: HardFault with sp=0x200007d0
    [tinygo: panic at /home/ayke/src/tinygo/tinygo/src/internal/task/task_stack_cortexm.go:48:4]

(This is with https://github.com/tinygo-org/tinygo/pull/3680 not yet
fixed and some local changes to configure the UART so I can actually see
the panic).

For atsamd21/nrf51 chips this results in a binary size reduction of
around 100 bytes. For other Cortex-M chips it's around 24 bytes but I
hope to change this in the future because a lot of the fault decoding in
runtime_cortexm_hardfault_debug.go should IMHO be done by the TinyGo
monitor instead (I estimate that this would save around 800 bytes on
these chips).

src/runtime/runtime.go

@@ -57,6 +57,12 @@ func memzero(ptr unsafe.Pointer, size uintptr)
 // the current stack pointer in a platform-independent way.
 func stacksave() unsafe.Pointer
 
+// Special LLVM intrinsic that returns the SP register on entry to the calling
+// function.
+//
+//export llvm.sponentry.p0
+func llvm_sponentry() unsafe.Pointer
+
 //export strlen
 func strlen(ptr unsafe.Pointer) uintptr