Improve zstd_opt build speed and size

[terrelln]

Use the same trick as we did for zstd_lazy in PR #2828:

• Create one search function specialization for each (dictMode, mls).
• Select the search function pointer at the top of the match finder.

Additionally, we no longer inline ZSTD_compressBlock_opt_generic into
every function, since dictMode is no longer used as a template. Create
two specializations, for opt levels 0 and 2, and call one of the two
specializations.

Lastly, remove the hack that disabled inlining for zstd_opt for the
Linux Kernel, as we've gotten most of the benefit already.

Compilation time sees a ~4x reduction:

Compiler	Flags	Dev Time (s)	PR Time (s)	Delta
gcc	-O3	10.1	2.3	-77%
gcc	-O3 -fsanitize=address,undefined	61.1	10.2	-83%
clang	-O3	9.0	2.1	-76%
clang	-O3 -fsanitize=address,undefined	33.5	5.1	-84%

Build size is reduced by 150KB - 200KB:

Compiler	Dev libzstd.a Size (B)	PR libzstd.a Size (B)	Delta
gcc	1327476	1177108	-11%
clang	1378324	1167780	-15%

There is a <2% speed loss in all cases:

Compiler	Level	Dev Speed (MB/s)	PR Speed (MB/s)	Delta
gcc	16	4.78	4.72	-1.25%
gcc	17	3.49	3.46	-0.85%
gcc	18	2.92	2.86	-2.04%
gcc	19	2.61	2.61	0.00%
clang	16	4.69	4.80	2.34%
clang	17	3.53	3.49	-1.13%
clang	18	2.86	2.85	-0.34%
clang	19	2.61	2.61	0.00%

Fixes Issue #2862.

[Cyan4973]

Great work ! Nice build time and binary size savings !