RFR: 8286056: AArch64: clarify uses of MacroAssembler::far_call/MacroAssembler::far_jump

[Evgeny Astigeevich]

`MacroAssembler::far_call`/`MacroAssembler::far_jump` are for generating calls of/jumps to targets which are inside `CodeCache` and can `relocInfo` types: `external_word_type`, `runtime_call_type` or `none`.
This PR updates the description of the functions and adds an assert checking a `relocInfo` type.
Tested a fastdebug build:
- `gtest`: Passed
- `tier1`: Passed

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Commit messages:
 - 8286056: AArch64: clarify uses of MacroAssembler::far_call/MacroAssembler::far_jump

Changes: https://git.openjdk.java.net/jdk/pull/8534/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8534&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286056
  Stats: 16 lines in 3 files changed: 13 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8534.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8534/head:pull/8534

PR: https://git.openjdk.java.net/jdk/pull/8534