ANDROID: arm64: patching: Add aarch64_addr_write()

The process of applying hypervisor relocations involves patching
addresses in the hypervisor object. In the existing KVM nVHE relocation
procedure, the relocations are applied early enough for write-permission
not to be a problem when touching e.g. the rodata section. But applying
relocations on hypervisor modules embedded in a kernel module proves
more challenging, as the kernel module loader will actively map text and
rodata sections read-only. In order to allow patching in those sections,
let's introduce a new helper function using the text fixmap to
temporarily map the relevant pages writable.

Bug: 244543039
Bug: 244373730
Change-Id: I9dcdade1927e5bc121db87bc950fb70a374c44cd
Signed-off-by: Quentin Perret <qperret@google.com>