RFR: 8254994: [x86] C1 StubAssembler::call_RT, "call_offset might not be initialized"

[Aleksey Shipilev]

Static analyzers complain `call_offset` might not be initialized in `StubAssembler::call_RT` in `c1_Runtime1_x86.cpp`.
The way I see it, it depends on `align_stack` value, and it is set whether it `align_stack` is `true` or `false`. But
we can probably make it cleaner so that future errors would be clearly detectable. Since it is initialized off
`offset()`, it should not be negative.

Testing:
 - [x] Linux x86_64 tier1

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Commit messages:
 - 8254994: [x86] C1 StubAssembler::call_RT, "call_offset might not be initialized"

Changes: https://git.openjdk.java.net/jdk/pull/730/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=730&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254994
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/730.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/730/head:pull/730

PR: https://git.openjdk.java.net/jdk/pull/730