[RISCV][LLVM] Enable atomics for 'Zalrsc' (#163672)

The 'A' atomics extension is composed of two subextensions, 'Zaamo'
which has atomic memory operation instructions, and 'Zalrsc' which has
load-reserve / store-conditional instructions.
    
For machines where 'Zalrsc' is present, but 'Zaamo' is not, implement
and enable atomics memory operations through pseudo expansion. Updates the
predication and lowering control to be more precise about which 'Zaamo'/'Zalrsc'
feature was truly requisite.
    
There will be no functional change to subtargets supporting 'A', while
allowing 'Zalrsc' only subtargets to utilize atomics at an increased code
footprint.

0 files changed, 0 insertions, 0 deletions