libm: add separate variables for math library compilation

[niewim19]

JIRA: PD-237

Description

Changes in Makefile files for separation of libm from libphoenix

Motivation and Context

Separation of files in phoenix-rtos/libphoenix#166 requires addition in Makefiles

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: compiled for: ia32-generic, imxrt1064, imxrt1176, imx6ull.
• Tested with draft submodule update:
  • libphoenix: separation of libm from libphoenix test phoenix-rtos-project#345

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work
  • libmphoenix: separate math.h from libphoenix (draft) libphoenix#166
• I will merge this PR by myself when appropriate.

[nalajcie]

obsolete