Issue 4510 preprocessor var to always use all logical cores

[sergey-dryabzhinsky]

Take 2 for issue #4510

[sergey-dryabzhinsky]

Add new preprocessor var for cflags -DZSTDCLI_AUTO_THREADING_LOGICAL

[sergey-dryabzhinsky]

Ready for review

[sergey-dryabzhinsky]

Here some testing:
1 system:
CPU: 6-core/12-thread i7-6850K
OS: Proxmox3/ Debian7 + container OpenVZ6 (4 cores setup)

cmd1: grep -P 'cores|siblings' /proc/cpuinfo - as expected shows physical cpu attribs

siblings        : 12
cpu cores       : 6
siblings        : 12
cpu cores       : 6
siblings        : 12
cpu cores       : 6
siblings        : 12
cpu cores       : 6

cmd2: grep -c processor /proc/cpuinfo - as expected, equal to container setup

4

cmd3: w/o patch zstd -vv -T0 /dev/zero - not expected, wrong number of cores

*** Zstandard CLI (64-bit) v1.5.7, by Yann Collet ***
Note: 2 physical core(s) detected
Compressing with 2 worker threads
--zstd=wlog=21,clog=16,hlog=17,slog=1,mml=5,tlen=0,strat=2
--format=.zst --no-sparse --block-size=0 --memory=134217728 --threads=2 --content-size
zstd: /dev/zero is not a regular file -- ignored

cmd4: w/o patch zstd -vv -T0 --auto-threads=logical /dev/zero - shows number as expected

 *** Zstandard CLI (64-bit) v1.5.7, by Yann Collet ***
Note: 4 logical core(s) detected
Compressing with 4 worker threads
--zstd=wlog=21,clog=16,hlog=17,slog=1,mml=5,tlen=0,strat=2
--format=.zst --no-sparse --block-size=0 --memory=134217728 --threads=4 --content-size
zstd: /dev/zero is not a regular file -- ignored

[sergey-dryabzhinsky]

With patch not working as expected.