openjdk.mk: make MEMORY_SIZE computation portable

[zzambers]

Fixes non-portable (bash specific) code causing errors in log on some systems (ubuntu):

...
18:43:25  /bin/sh: 1: [[: not found
18:43:25  /bin/sh: 1: [[: not found
18:43:25  expr: syntax error: unexpected argument '1024'
18:43:25  /bin/sh: 1: [: Illegal number: 
18:43:25  expr: syntax error: unexpected argument '2048'
18:43:25  expr: syntax error: missing argument after '>'
...

testing: linux_x86-64 OK

[smlambert]

sparcv9_solaris JDK 8: https://ci.adoptium.net/job/Grinder/10132 x86-64_windows JDK 21: https://ci.adoptium.net/job/Grinder/10133

Also check a couple other platforms that often have different versions of tools installed ( make / bash )

[sophia-guo]

testing: linux_x86-64 OK

This is the same one with the error message and use the branch openjdk-hotspot-test, not the openjdk-memsize-fix

[zzambers]

testing: linux_x86-64 OK

This is the same one with the error message and use the branch openjdk-hotspot-test, not the openjdk-memsize-fix

Oh, you are right, I have accidentally used link to to grinder run with error twice.

Testing supposed to link this one: https://ci.adoptium.net/view/Test_grinder/job/Grinder/10097/

I will update description

[zzambers]

sparcv9_solaris JDK 8: https://ci.adoptium.net/job/Grinder/10132 x86-64_windows JDK 21: https://ci.adoptium.net/job/Grinder/10133

Also check a couple other platforms that often have different versions of tools installed ( make / bash )

I have only ran linux since modified code is in:

ifeq ($(OS),Linux)
...
endif

but probably better to test other systems anyway