mpl: pass MPL's CFLAGS back to MPICH

[yfguo]

Pull Request Description

PR#7074 consolidated SSE2 and AVX related optimization options into MPL's configure because only MPL explicitly use them. MPICH's CFLAGS no longer has those options. However, with Intel compilers, this does results in some performance degradation. Therefore, we need to propagate the CFLAGS in MPL back into the MPICH configure.

This PR creates a MPL_CFLAGS in the localdefs that allows the PAC_CONFIG_MPL macro to add it to the main CFLAGS.

This is an alternative and probably better solution than the 2nd commit in #7150 .

Author Checklist

• [ ] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
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• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
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[yfguo]

test:mpich/ch4/ofi test:mpich/ch4/ucx

[yfguo]

test:mpich/ch4/ofi test:mpich/ch4/ucx