[PowerPC] Fix LLD10 linking of ofw loader on ppc

[bdragon]

Summary

LLD and bfd, without this change, will link the ofw loader very differently. LLD creates several PT_LOAD sections with varying permissions, which OF does not grok.

Like we do for the kernel, specify the program headers manually.

Also, to work around a crash in ld.bfd, explicitly assign the section output to the text section. (LLD doesn't care, but our ld.bfd will outright crash.)

This removes the GNU_STACK header as well. This is harmless because we are using a statically-allocated stack, and OpenFirmware does not honor it anyway.

[jhibbits]

"LLD and bfd will link the ofw loader very differently." In what way? Please expand on this.

[bdragon]

Without this change, lld10 will create a binary with *several* PT_LOAD sections, which ofw does not understand.

With this change, both linkers create an acceptable output.

[fbsd-phab_maskray.me]

Looks good.

stand/powerpc/ofw/ldscript.powerpc

18 ↗

(On Diff #68624)

This uses the rule that program headers are propagated to SHF_ALLOC sections, so .hash will be added to text as well.