Default to boringssl when building on darwin.

[bzbarsky-apple]

This gets rid of the openssl dependency.

[github-actions]

PR #24929: Size comparison from d6bb8c2 to 1b2e0d0

Decreases (1 build for cc32xx)

platform	target	config	section	d6bb8c2	1b2e0d0	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20194648	20194647	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	d6bb8c2	1b2e0d0	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	640233	640233	0	0.0
			(read/write)	204084	204084	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197488	197488	0	0.0
			.comment	194	194	0	0.0
			.data	1476	1476	0	0.0
			.debug_abbrev	928439	928439	0	0.0
			.debug_aranges	87352	87352	0	0.0
			.debug_frame	299840	299840	0	0.0
			.debug_info	20194648	20194647	-1	-0.0
			.debug_line	2649781	2649781	0	0.0
			.debug_loc	2785870	2785870	0	0.0
			.debug_ranges	280720	280720	0	0.0
			.debug_str	3001804	3001804	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105585	105585	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	375840	375840	0	0.0
			.symtab	255856	255856	0	0.0
			.text	532524	532524	0	0.0