RFR: 8294087: RISC-V: RVC: Fix a potential alignment issue and add more alignment assertions for the patchable calls/nops [v6]

[Xiaolin Zheng]

> With RVC turning on, we must carefully keep all runtime-patchable instructions aligned. Code is running at full speed, when patching unaligned instructions spanning cache lines, concurrency issues may occur. This patch fixes a potential alignment issue of the patchable nop after MachUEPNode, with adding some strong alignment assertions as well. (In fact, currently the `nop` in the Verified Entry Point is fortunately aligned to 4 under RVC even without this patch, so this patch doesn't change program behaviors.)
> 
> Tested hotspot tier1 and tier2 together with other patches on QEMU.

Xiaolin Zheng has updated the pull request incrementally with one additional commit since the last revision:

  Revise as to comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10370/files
  - new: https://git.openjdk.org/jdk/pull/10370/files/a8cede0f..5e54d5dd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10370&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10370&range=04-05

  Stats: 17 lines in 7 files changed: 1 ins; 8 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/10370.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10370/head:pull/10370

PR: https://git.openjdk.org/jdk/pull/10370