Change ARM code to use position independent code

This is all arising from issue #68 and PR #76.  It seems that MacOS Arm64
does not like the use of relocation in the original switch-arm64.c code.
This arises from a single instruction
    ldr lr, =action_entry

As action_entry is extremely close to this code this can simply be replaced
with
    adr lr, action_entry
for both ARM32 and ARM64.

Author: Araneidae

context/switch-arm.c

@@ -68,7 +68,7 @@ FSIZE(switch_frame)
 FNAME(create_frame)
 "       stmfd   r0!, {r1, r2}\n"       // Save arguments for new coroutine
 "       mov     ip, lr\n"              // Save LR so can use same STM slot
-"       ldr     lr, =action_entry\n"
+"       adr     lr, action_entry\n"
 "       stmfd   r0!, {r4, r5, r6, r7, r8, r9, sl, fp, lr}\n"
 IF_VFP_FP(
 "       fstmfdd r0!, {d8-d15}\n")

context/switch-arm64.c

@@ -105,7 +105,7 @@ FSIZE(switch_frame)
 FNAME(create_frame)
 "       stp x1, x2, [x0, #-16]!\n"
 "       mov x16, lr\n"               // Save LR so can use same STP slot
-"       ldr lr, =action_entry\n"
+"       adr lr, action_entry\n"
 "       stp x19, x20, [x0, #-16]!\n"
 "       stp x21, x22, [x0, #-16]!\n"
 "       stp x23, x24, [x0, #-16]!\n"