RFR: 8267098: AArch64: C1 StubFrames end confusingly

[Alan Hayward]

For many of the stub frames, a leave/ret is generated after the stub has
already branched or returned. This is confusing. For these cases, replace
the superfluous code with a should_not_reach_here

For handle excception, instead of storing return from the exception
handler on the stack, it can be moved directly into lr, replacing a store and
load with a single move. (If/when PAC support is implemented, then this store
would also have to be signed).

-------------

Commit messages:
 - AArch64: Improve C1 Stub Frame code

Changes: https://git.openjdk.java.net/jdk/pull/4030/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4030&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267098
  Stats: 52 lines in 1 file changed: 14 ins; 16 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4030.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4030/head:pull/4030

PR: https://git.openjdk.java.net/jdk/pull/4030