Cleanup x86_patch and amd64_patch. (mono/mono#16474)

Cleanup x86_patch and amd64_patch. (mono/mono#16474)

* Cleanup x86_patch and amd64_patch.

amd64_patch has two incorrect looking probably unused cases.
 g_assert(0) and fix them.
 call and jmp indirect (one case)
 move reg RIP relative

x86_patch is large-ish inline macro.
Presumably for sharing with amd64, not inlining.
Restructure as an inline function, called once.
Try to make the code a little clearer also, with same functionality and efficiency
 (modulo the optimized perf of "if i == 4" vs. "if i")
i.e. speak of instruction_size and offset_size, not pos and boolean size.

Add to x86_patch the ability to patch jmp/call indirect,
as amd64_patch has, but which was incorrect.
This would seem useful for a later change (using "thunks"
to patch nearby data instead of patching aligned instructions,
indirect call/jmp vs. direct).

amd64_patch largely delegates to x86_patch.
One case asserts and delegates.
Move the assert to the x86 side, where an optimizing
compiler should know to remove it for x86 anyway.

Change some assert to g_assert.

Remove some unnecessary casts.

Fix an assert (https://github.com/mono/mono/pull/16471).

* Change g_assert back to assert except where making larger changes
and less expansion.

Commit migrated from https://github.com/mono/mono/commit/f735571f27738e3354f6d92650a0517d0482039d