ANDROID: arm64: patching: Refactor __aarch64_insn_write()

In order to allow the re-use of __arch64_insn_write() to fixup
relocations in pKVM modules, refactor it into a more abstract
__arch64_text_write() function which also takes a size as parameter.

No functional changes intended.

Bug: 244543039
Bug: 244373730
Change-Id: I60d789d8a4b1271deeb2f6ac6d3b7fc55bdbb465
Signed-off-by: Quentin Perret <qperret@google.com>