[6.2][Runtime] Add function_cast, switch from std::bit_cast.

Function types aren't always trivially copyable, e.g. with address-discriminated signed pointers on ARM64e. Introduce a function_cast helper and use that instead.

(cherry picked from commit 185b739cf1)

test/embedded/dependencies-concurrency.swift

@@ -18,6 +18,7 @@
 // DEP: _exit
 // DEP: _free
 // DEP: _malloc
+// DEP: _memcpy
 // DEP: _memmove
 // DEP: _memset
 // DEP: _memset_s