[arm][ios] Follow branch islands when determining method entry addresses. (#33182)

[arm][ios] Follow branch islands when determining method entry addresses. (#33182)

Fixes https://github.com/mono/mono/issues/18612; `aot_code_low_addr` and
`aot_code_high_addr` were set to a range that included only first-level
branch islands, so `find_aot_module` would fail early when passed a
genuine function address.

This change only affects monotouch for now.

Why not use relative offsets stored in a read-only section? One reason
is that llvm-as cannot handle a `.long` directive containing a
subtraction expression involving externally-defined symbols. The AOT
method address table and AOT LLVM-generated code are currently emitted
in separate object files, so the method address table would include
expressions with undefined symbols.

Related: https://xamarin.github.io/bugzilla-archives/70/707/bug.html

Co-authored-by: imhameed <imhameed@users.noreply.github.com>