Static OS check to select clock

[bitfaster]

This brings a more noticeable gain, building on #540. The OS check is not a JIT intrinsic until .NET8, so repeated checks are a penalty. As a follow up, need to verify whether windows bench was regressed and whether the OS check is ultimately elided by the JIT.

With this fix (static readonly bool):

Method	Mean	Error	StdDev	Ratio	Allocated
ConcurrentDictionary	4.023 ns	0.0114 ns	0.0089 ns	1.00	-
FastConcurrentTLru	14.995 ns	0.0046 ns	0.0036 ns	3.73	-
FastConcLruAfterAccess	17.534 ns	0.0465 ns	0.0388 ns	4.36	-
FastConcLruAfter	20.035 ns	0.0061 ns	0.0051 ns	4.98	-
ConcurrentTLru	19.744 ns	0.0047 ns	0.0037 ns	4.91	-

v2.5.0 baseline

Method	Mean	Error	StdDev	Ratio	Allocated
ConcurrentDictionary	3.935 ns	0.0771 ns	0.0721 ns	1.00	-
FastConcurrentTLru	16.610 ns	0.0064 ns	0.0054 ns	4.22	-
FastConcLruAfterAccess	18.822 ns	0.0069 ns	0.0058 ns	4.79	-
FastConcLruAfter	21.149 ns	0.0166 ns	0.0139 ns	5.38	-
ConcurrentTLru	21.449 ns	0.0067 ns	0.0052 ns	5.46	-

BenchmarkDotNet v0.13.12, macOS Sonoma 14.1.1 (23B81) [Darwin 23.1.0]
Apple M2, 1 CPU, 8 logical and 8 physical cores
.NET SDK 8.0.100
  [Host]   : .NET 6.0.29 (6.0.2924.17105), Arm64 RyuJIT AdvSIMD
  .NET 6.0 : .NET 6.0.29 (6.0.2924.17105), Arm64 RyuJIT AdvSIMD
Job=.NET 6.0  Runtime=.NET 6.0  

[bitfaster]

Confirmed branch based on static variable is elided for .NET6.

Baseline:

.NET 6.0.29 (6.0.2924.17105), X64 RyuJIT AVX2

; BitFaster.Caching.Benchmarks.TimeBenchmarks.EnvironmentTickCount64()
;             return Environment.TickCount64;
;             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       jmp       near ptr System.Environment.get_TickCount64()
; Total bytes of code 5

Extern method
System.Environment.get_TickCount64()

With static bool, comparing windows disassembly:

.NET 6.0.29 (6.0.2924.17105), X64 RyuJIT AVX2

; BitFaster.Caching.Benchmarks.TimeBenchmarks.DurationSinceEpoch()
;             return Duration.SinceEpoch();
;             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       sub       rsp,28
       call      System.Environment.get_TickCount64()
       nop
       add       rsp,28
       ret
; Total bytes of code 15

Extern method
System.Environment.get_TickCount64()

Without static bool (no branch, original code):

.NET 6.0.29 (6.0.2924.17105), X64 RyuJIT AVX2

; BitFaster.Caching.Benchmarks.TimeBenchmarks.DurationSinceEpoch()
;             return Duration.SinceEpoch();
;             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       sub       rsp,28
       call      System.Environment.get_TickCount64()
       nop
       add       rsp,28
       ret
; Total bytes of code 15

Extern method
System.Environment.get_TickCount64()

[coveralls]

coverage: 99.318%. remained the same
when pulling 02c676f on users/alexpeck/macos2
into 4c91b83 on main.