[clang] [llvm] PowerPC: Rename "float-abi" module flag to "long-double-type" (PR #210817)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> GCC uses `.gnu_attribute` asm to mark which long double type an object uses, which is also the way compiler identifies soft/hard floating point config. Will it be inconsistent between backend and frontend if we use separate abi values for different float categories?

No, this is an IR change only. The backend has to route the corresponding module flag to whatever the right .gnu_attribute is 

https://github.com/llvm/llvm-project/pull/210817