MIPS32: add .gnu.attributes and .MIPS.abiflags sections to the output object

[wzssyqa]

MIPS32 uses .gnu.attributes and .MIPS.abiflags to mark the ABI that
the object is using, and the kernel will set the correct FP mode for it.

Currrently we have no these 2 sections and thus since kernel will
treat the objects as FP32. To support FPXX and FP64 in future,
we should add support of these 2 sections.

More details about FP32/FPXX/FP64 are explained in:
    https://web.archive.org/web/20180828210612/https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-_FR0_and_FR1_Interlinking

[wzssyqa]

this bug blocks #39289
MIPS32: switch to fpxx from fp32 or even fp64

[wzssyqa]

I figure out a WIP patch:
https://github.com/wzssyqa/go/tree/add-section
RFC, and more test is needed.
Once we have more test, I will submit a CL.

[gopherbot]

Change https://golang.org/cl/239217 mentions this issue: cmd/link: mips32, add .gnu.attributes and .MIPS.abiflags for internal-ld

[mengzhuo]

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