Add new JIT stress modes to the CI system; clean up netci.groovy duplication

Add new JIT stress modes to the CI system; clean up netci.groovy duplication

1. Add JIT tailcall stress: COMPlus_TailcallStress=1
2. Add "SSE2 only": COMPlus_EnableAVX=0, COMPlus_EnableSSE3_4=0
3. Add new JIT stress regs mode: COMPlus_JitStressRegs=0x1000
   a. Do this plain, with JitStress=2, with CoreFX, and in R2R testing
4. Fix R2R JitStressRegs10 and 80 to actually test 10 and 80! (previously, they
   were passing "x10" and "x80", which get parsed as zero.
5. Delete tons of duplicate code from netci.groovy, and use a table/function
   to determine test display names.

Fixes dotnet/coreclr#8626, dotnet/coreclr#6129, dotnet/coreclr#10004

Commit migrated from https://github.com/dotnet/coreclr/commit/1aaa378a37c97705fee92df7c1450503aee2a346