csu: Find the main pointer through the GOT

Use the Global Offset Table to find the location of main in crt1. With
lld the old code would point to main@plt, however ld.bfd fails to link
when main is in a shared library.

Fix this by using the GOT address to find main as it works with both
lld and bfd.

Reviewed by:	jrtc27
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D45259

lib/csu/aarch64/crt1_s.S

@@ -44,8 +44,8 @@ ENTRY(_start)
 	add	x2, x1, x0, lsl #3 /* env is after argv */
 	add	x2, x2, #8	/* argv is null terminated */
 #ifdef PIC
-	adrp	x4, main
-	add	x4, x4, :lo12:main
+	adrp	x4, :got:main
+	ldr	x4, [x4, :got_lo12:main]
 #else
 	ldr	x4, =main
 #endif