YJIT: Add compilation time counter

[k0kubun]

This PR adds a counter of cumulative time taken for compiling JIT code. Similar to GC.stat(:time), if you call this before and after your request, you can measure how long you spent in YJIT compilation for each request.

When we compare multiple Ruby versions, we could monitor GC.stat(:time) and this to check how much impact each of GC and YJIT had on their difference in performance.

Example

benchmark	compile_time_ms
30k_ifelse	1,234ms
30k_methods	429ms
railsbench	621ms
lobsters	1,699ms

[maximecb]

Once the switch to Instant is made, could you also measure if there is any impact on total run time for the first iteration of e.g. 30k_ifelse. I would expect not, but just want to validate that this doesn't slow down benchmarks due to context switches. Always good to double-check.

[k0kubun]

could you also measure if there is any impact on total run time for the first iteration of e.g. 30k_ifelse

I ran one iteration of the following benchmarks:

--yjit

before: ruby 3.3.0dev (2023-09-11T17:09:22Z master 1ae5dd6f32) +YJIT [x86_64-linux]
after: ruby 3.3.0dev (2023-09-12T16:02:16Z yjit-compile-time 42b9e18d96) +YJIT [x86_64-linux]

-----------  -----------  ----------  ----------  ----------  -------------  ------------
bench        before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
lobsters     1738.2       0.0         1722.0      0.0         1.01           1.01
railsbench   1498.7       0.0         1493.5      0.0         1.00           1.00
30k_ifelse   1345.2       0.0         1327.1      0.0         1.01           1.01
-----------  -----------  ----------  ----------  ----------  -------------  ------------

--yjit-stats

before: ruby 3.3.0dev (2023-09-11T17:09:22Z master 1ae5dd6f32) +YJIT [x86_64-linux]
after: ruby 3.3.0dev (2023-09-12T16:02:16Z yjit-compile-time 42b9e18d96) +YJIT [x86_64-linux]

----------  -----------  ----------  ----------  ----------  -------------  ------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
lobsters    1918.4       0.0         1909.9      0.0         1.00           1.00
railsbench  1606.6       0.0         1603.7      0.0         1.00           1.00
30k_ifelse  1725.2       0.0         1705.3      0.0         1.01           1.01
----------  -----------  ----------  ----------  ----------  -------------  ------------

This PR doesn't seem to impact the 1st itr performance whether this feature is enabled or not.

[maximecb]

Very nice 👌