Readd documentation for 32-bit ARM -march and -mfpu

[efriedma-quic]

The clang manual had documentation at one point, added by https://reviews.llvm.org/D73459 / ac21535 , but it got deleted because it was added to an automatically generated file. Might be worth looking into adding it back.

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Eli Friedman (efriedma-quic)

The clang manual had documentation at one point, added by https://reviews.llvm.org/D73459 / ac21535 , but it got deleted because it was added to an automatically generated file. Might be worth looking into adding it back.