intelhex: Increase optimisation level to O3.

[microbit-carlos]

Optimisation only applied to the intelhex.c file.

In my tests with micro:bit V2, when build with GCC v10.3, this can improve flashing time (rsync a .hex file with 512Kbs of flash data) by around 0.7-0.8 seconds.

For additional info, building the entire project with -O3 shaved another second-ish of flashing time, but that might be too much of a change/risk (it's likely it can also increase binary size, but I didn't really checked those numbers). I couldn't easily find another single file in the USB or vfs layer that significantly improved times on their own, just a tiny bit here and there, but the intelhex.c file is significant enough where this might be worth doing.

The specific compiler incantations have been copy/pasted from the SW_DP.c file:

DAPLink/source/daplink/cmsis-dap/SW_DP.c

Lines 31 to 38 in 9e32454

	#if defined(__CC_ARM)
	#pragma push
	#pragma O3
	#pragma Otime
	#elif defined(__GNUC__) && !defined(__ARMCC_VERSION)
	#pragma GCC push_options
	#pragma GCC optimize("O3")
	#endif